Ulcer In Gum, Swelling, Painful, Had Wisdom Tooth Extracted. Had Ibuprofen. What Is Wrong?

I have a ,what appears to be ulcer, on my back gum behind molar, its very swollen to the point were i cant close my mouth and my gum is like over my tooth because of swelling, also very painful when i do try to close my mouth,i can barley chew food, it was throbbing earlier but it went away after ibuprofen, i get ulcers frequently but never one there, just want to make sure nothing serious that i need to go to dentist right away I noticed it for the 1st time sunday night,so almost 5ish days and also had wisdom teeth out two years ago so i don't think its related to that