No one is going to be able to diagnose you over a forum im afraid (plus users arent allowed to really either). Didn't your GP give you any more info when he gave you the script and looked you over..?
To your first bit though, i doubt its an absess without any pain, those things tend to hurt like an absolute ***** when theyre in that area. Might simply be, as you noted, an ulcer thats been further damaged/inflamed? Either way, it would be better to let your doctor/dentist examine it than soliciting advice on the internet as it will likely be wrong
Dr google being equally unhelpful.