Is a cyst or bump on the gums always a tooth infection?

I have had a small, clear, painless blister or cyst on my lower inner gum, close to my last molar that keeps coming back. It started as a tiny bump that grew slowly in several days and became ever so slightly irritated. I ended up popping it with my finger (maybe slightly salty tasting). The next day it grew back to a tiny bump. The area around it is also a bit redder than the other side. But I have no pain in any teeth. I went to the dentist (after not going for years), worried about an abscess. He tried to look at the area, but couldn't see much and did not seem concerned -- said things look normal to him. But, at the time it was only minimally flared up. He did x-rays and full exam and said everything looked good with my teeth and fillings. His only diagnosis was that I most likely grind my teeth at night. But, I don't see how that can be related to this? It's an area behind all teeth. It will go flat after I pop it, but then keeps coming back a day (or 2 days) later (had it for 3+ weeks now). It will sometimes even seem to go away almost completely for a couple days, but always comes back. It still doesn't hurt, but when it's back it's very noticeable and bothersome when my tongue hits it. Could the dentist have missed an infection of some kind? Should I get a second opinion from another kind of Dr?