My left trap is a lot bigger than my right one but I've never done any trap building exercises, what could be wrong?

Muscle atrophy. It may be that the larger side is normal and the smaller side is the abnormal side or atrophic. You should be evaluated by an orthopaedic surgeon to make sure the source of the atrophy is not coming from compression in the cerical spine that is affecting the motor nerve to the smaller trapezius.
Hard to say. There could be swelling in that region, for some reason, or, possibly a mass of some sort. An orthopedist should start with an xray and an exam, to begin to find the origin.