Q. Chinese dwarf hamster is 3 yrs old and Peanut has gotten a black lump in between his tooth and gums(inside mouth). Do you have any idea of what it is?
A. Hamster have little cheek pouches, where they store and carry food. Any chance his food has sunflower seeds in it? I bet that may be what you are seeing.
If you watch him when he eats, you may notice him tuck away a little snack for later-he didn't swallow it whole, its in his cheek.
If you keep noticing it though, I would take him to the vet. He should remove seeds but on occasion things can get stuck in there and start an infection.