Im thinking of multiclassing my level 3 barbarian (Path of the Ancestral Guardian) into fighter, but before I do, I was wondering. After you multiclass, can you choose to level your original class again? This might be a dumb question, but I'm worried that I multiclass into fighter, and I can't level my barbarian anymore. 

Yes you can. Each time you level up you choose if you want to pick a level of your original class or one of the classes you have multiclassed to. 

Yes, you can level in any class you qualify for at any time.