What Could Small, Hard Lump Behind Upper Molar Indicate?

I have a very small, hard lump, behind the last upper molar on the right side of my mouth. Part of it is sticking out of my gums, but I cannot see it. I don't Have a small enough mirror. If I mess with it, the gums swell up, and I have an unpleasant taste in my mouth. I told my wife, and she asks, "do you have mouth cancer?" I sarcastically replied, "wow, I hope so!" I am going to schedule a long-overdue dentist appointment, but am looking for "words of wisdom". Pretty much painless though. Is it common for this to occur? I've since lost my appetite. Please let me know your thoughts on the matter.
Thanks