I just wanted to put my 2 cents in and tell everyone who might be reading this (and feels like they didn't get a straight answer from all the posts) what we found out when both of our daughters had this same kind of issue with their baby teeth:
They were abscessed teeth. They both had to have the teeth pulled (but for adult teeth, they usually opt for a root canal first to save the tooth).
My husband and daughter both get canker sores (which we found is from the SLS in toothpaste/mouthwash), and this was NOT IT. They do look similar so I can see where people get confused or they don't get checked out sooner. But when compared to canker sores, this looks different. I wish I had pictures to show you. Canker sores aren't as bright of a white, at least in what I've seen. The pimple like thing from an abscessed tooth can easily be mistaken for a tooth erupting from the gums. Canker sores simply look like sores.
Abscessed teeth get that little pimple when they are draining, and when they drain there is little to no pain. When people do not feel pain, they feel less concerned, etc. But with an abscessed tooth, you need to get it checked out right away even if it is draining/painless because the bacteria can get into the blood stream and in worst case scenario cause death.
The things to look for if you just don't know and cannot get to a dentist right away are signs of infection, including fever, nausea, and a swollen jaw or neck and/or swelling around the tooth/gums. Other symptoms include pain when chewing, sensitivity to hot/cold, sore throat, bitter taste in the mouth. Also, one of my daughters had bad breath for a while that we couldn't figure out. It was there even right after she brushed.
In a way, you are extremely lucky not to have the severe pain of an abscessed tooth (which I understand is one of the worst things to experience). But at the same time, it is rather unlucky because that tricks you into not getting checked out.