You have been bitten by a lycanthrope of some nature wether it be an evil were-wolf, a stalwart were-bear, a were-tiger, etc. and as such if you do not resist the curse of lycanthropy you are now cursed and may now morph your physical form into that of a half human, half were form or into your fully transformed state. When you attack another humanoid creature they must make and succeed on a constitution saving throw (DC 8 + the lycanthropes prof bonus + the lycanthropes constitution modifier. While you are in animal form you cannot speak. A lycanthrope gains proficiency with their natual attacks, and you gain the alignment, and movement speed asscociated with your were form. Please see the PC lycanthrope pages in the monsters manual and ask your DM about resistances and immunities associated with each form.
Werbear: The character gains a strength of 19 if your score wasnt already there or higher, and your ac increases by 1 when in you hybrid or bear form. (from natural armor) and your attacks made with your natural weapons are made with the strength modifier.
wereboar: The character gains a strength score of 17 if their score isnt already higher, and a plus one bonus to your AC while in your hybrid or boar form. the attack and damage rolls made for your natural tusk weapons are made with the strength modifier. for the charge trait the DC is 8+the characters strength modifier+ prof bonus.
wererat: the character gains a dexterity score of 15 if there scoer isnt already higher. attack and damage rolls made for the bite trait are made with whichever is higher the strength or dexterity modifier.
weretiger: the character gains a strength score of 17 of their scoere isnt already higher. Attack and damage rolls made for the natural weapons are based on your strength modifier. for the pounce trait the DC is 8 + your strength mod + your prof bonus.
werewolf: the character gains a strength score of 17 if it isnt already higher, and a +1 bonus to AC while in hybrid or wolf form (from natural armor). attack and damage rolls for your natural weapons are based on the strength modifier.