I had a ball with a similar look recently, but worse. In my case it was an infection caused by the snake biting a piece of bedding when eating a rat. It appeared at the tooth line to start,in a day moved to upper gums and lip and by the next morning the whole head on that side was swollen. Time frame was about 36-48 or so hours to get that bad, but once the swelling started, it took off.
Take your snake to a vet. They will tell you what it is for sure. Let them do the prying open of the mouth so it doesn't have it's mouth poked and pryed 10 times over between you and them combined. If it is nothing, great, but if it is an infection, then you want to start treatment before it gets bad. Mine ended up with swelling into the neck before the drugs started to show effect.
If it's an infection most likely they will give you baytril or something for antibiotic, and possibly something to swab on the spot to clean if they feel it's needed. You want to head it off before it gets bad.
Hopefully it will be nothing, but in any case best of luck to you and your ball!