Small, Hard, Painful Bump On My Gums Under Teeth?

There is a very small but very, very hard bump that is slightly painful on the gums underneath my bottom teeth, on the back side. It has been there for four or five days.

I have had little bumps and clear cysts in my mouth before, but nothing ever hurt, was this hard, or lasted this long before.

I tried popping it, but it's really hard and hurts a lot.

What could it be?