Does anyone know what this is? Should I be worried?

So this appeared in my mouth for the first time a while ago, no reason at all, i thought i had bit my cheek but then it appeared again about 2 days after making out with a guy (about 6 months ago) and now again last week a day after making out with my boyfriend which i found very wierd because we always do that. it goes away after a couple of hours a day max and feels like a little ball