The official medical term for "Singaw" is mouth ulcer, which sounds worse than it really is. Americans also use the term Canker Sore or Cold Sores, probably just to spite the British for laying the claim of naming one of the most common oral annoyances next to uttering the word "actually".
Singaw is derived from the local idea that when heat builds up in the mouth, it has to release it somewhere, causing the lips to bloat and eventually develop a hole from where heat is expelled. I'm guessing somebody teaching a class of students accidentally switched his geology manual with the one for biology, so now we have to deal with volcanoes in our mouths.
All things considered, there is no real quick cure for singaw. I should know. I get these sores at least once every two months, more often if the weather keeps on switching from blistering hot to nipple-chillingly cold. It happens so often I wouldn't be surprised to see one day a singaw developing inside a singaw. I should go get surgery and attach a zipper to my lips. When a singaw is about to form, I'd just unzip that motherfucker. "There you go, minivolcano."
That said, having a singaw is not fun. You sound like an idiot when talking and trying to avoid hitting the sore at the same time, and you can only eat certain types of food without ending up twitching on the floor in agony. Of course people will always act like they've never had singaw before when they ask you what's in your mouth.
To which, it's always interesting to say, "Oral Herpes. Wanna kiss?"