So, I'm playing a campaign with one of my friends, and my character is level 8, I've got 5 levels into Paladin(starting class), and 3 into Warlock, and everything was fine until I got to level 3 warlock, as soon as I finished up my warlock level three pact boon for the tome, I lost my 1st level pact slots, why did this happen? Any help would be great, some questions I have are, is this part of the class? Am I doing something wrong? Is DND beyond just trolling me? How do I fix it?

Your pact slots should now be 2nd level. As a warlock you have 2 level 2 spell slots at level 3. Link to character sheet?

Warlocks Spells slots works differently than other spellcasters.

A Warlock Pact Slots isn't an assortement of slots like other classes, there's no 3 lvl 1 slots, 2 lvl 2 slots and 1 lvl 3slots etc.

You have 2 Level 3 slots( then a 3rd slot at 11th lvl as a Warlock and the 4th at 17th lvl Warlock)

All your Pact Magic slots will "grow" in power, meaning that a lvl 17 warlock for example has 4 lvl5 Spell slots.

Wich also means that no matter the spell, you'll cast it as a spell slot of that lvl, wich sadly also means that you'll have to use higher spell slots to cast even spells that doesn't upscale( your lvl 6 Warlock wants to cast detect magic?, yeah you're still gonna use that 3rd lvl slot anyways...)

Now since you MCed from Paladin, the advantage is that you still have the 1st and 2nd lvl slots from your paladin.

So you can use those lower lvl slots to cast low level utility/support spells, and keep the Pact Slots for higher spells that deals damages or for your Smites.

Also Per the MC rules for Warlocks and Paladin, you can use your Paladin slots for casting your Warlock spells, and vice versa, and you can use your Pact slots to use your Divine Smite.

Normaly on your sheet if you scroll down enough, in the spells, you'll see that the Pact slots are separated from the other slots of the same level.

https://www.dndbeyond.com/profile/MM_0211/characters/22832045 here's my sheet, I think I understand the fact that my slots just get bigger, and don't get more if them, does that mean that every slot I get will simply be a higher level every time I level up? Like, do I simply get slots for higher levels and none of that lower level stuff, like, wizards get more slots, do warlocks keep the number if slots just for better spells?

Quote from MM_0211 >>

https://www.dndbeyond.com/profile/MM_0211/characters/22832045 here's my sheet, I think I understand the fact that my slots just get bigger, and don't get more if them, does that mean that every slot I get will simply be a higher level every time I level up? Like, do I simply get slots for higher levels and none of that lower level stuff, like, wizards get more slots, do warlocks keep the number if slots just for better spells?

 This is all described in the SRD, the Player's Handbook, and here on D&D Beyond.

Quote from MM_0211 >>

https://www.dndbeyond.com/profile/MM_0211/characters/22832045 here's my sheet, I think I understand the fact that my slots just get bigger, and don't get more if them, does that mean that every slot I get will simply be a higher level every time I level up? Like, do I simply get slots for higher levels and none of that lower level stuff, like, wizards get more slots, do warlocks keep the number if slots just for better spells?

 Yes.

Your Pact Slots(and only them) will grow stronger up to lvl5 slots, and the max of slots a Pure/lvl17 Warlock can have is 4.

Now you can always use Higher lvl slots to cast lower level spells, its just that in some cases when a spell doesn't upscale, its kinda wastefull( thats why having the lower level slots from the paladin levels is a huge boon in this case)

Now from lvl 11 a Warlock gets the Mystic Arcaneum feature, and again at 13th 15th and 17th level.

Those features gives you a selection of spells from higher levels that you can Use ONCE per long rest, and they don't use Spell Slots of any kind.

Its the Warlocks way to have lvl 6-7-8-9 spells, (but only ONE of each, respectively at lvl 11-13-15-17)when you get the Feature at the appropriate level, you get a selection of spells of the appropriate levels to choose from and you then can use them Once per Long rest without a Spell slot.

Its a weird roundabout for Warlocks to have higher level spells, without them having spell slots higher than 5th lvl.

Unfortunatly, and if i'm not mistaken, contrary to your other spells in your spell list, you cannot change the spells you gained with Mystic Arcaneum when you level up or through the Spell Versatility feature from the UA Class features variants.

Once a Mystic Arcaneum has been choosed, its permanent, you can off course talk about it with your DM to be able to change the spells once every so often or like when using the Spell versatility UA, cause honestly, its a part of the feature that sucks...

That is off course if you plan to get more Warlock levels than Paladin levels.

Now after taking a look at your character sheet i'm curious why with your ASi you did take a point in INT?

You have 15 in INT, so you still only have +2 MOD in it for your skills, so why not put that point into WIS instead to have at least a +1 to your Perception and Insight skills?

Also when you take the Hexblade Patron, in your weapons in your inventory gets a new pair of options "Hex Weapon" ticking it will switch the Ability Mod used on your sheet to CHA for this weapon.

Wich in your case since you have +5 mod in CHA would put your to Hit rolls with your Greatsword at +9 and the damage at +6.

Also you should check your gear, cause technicaly you have Both your Plate armor AND your chainmail equipped at the same time.

Now the app will only take the highest AC into consideration, so no worries about that, but in the case where you get multiple armors with different enchantements on it, it can get problematic.

And Gods do you guys carry an ungodly amount of gear..., you're all armed to the teeths!