A week ago during the evening I had a swelling behind my tooth #18, ie. the swelling was "above" tooth #17, which isn't showing in my mouth:
I couldn't close my mouth completely in that side without severe pain (couldn't get the upper teeth to touch the lower ones) and as a result mumbled a bit. It remained swollen through the night, but then suddenly disappeared before noon the next day (coincidently 5 mins before a job interview

). I have not had my wisdom teeth removed or anything. What could this be/something to be worried about?