Help! My hamster has a lump

Question

Hello,
Yesterday, when I was handling my longhaired black Syrian hamster, Sooty, that he has a lump just behind his left armpit. At closer inspection, I saw that it is black, has a slightly shiny look, is not covered in hair, is perhaps 4mm in length, and is raised quite a bit from his body. I worried it is a tumour or something like that. It's not affected his behaviour at all, and is still eating/drinking etc. It doesn't seem tender at all, he doesn't flinch or anything when I touch it.
Could you tell me what it could be? Is it serious. I'm just so scared for him, I couldn't bear to lose him.
Thank you so much,
Kathryn

Answer
Hi Kathryn,

    It is very possible that it could be a tumor. It could also be an abscess, although these are generally a pinkish red or skin color. Either way I would suggest having him seen by a vet.
    If you have anymore questions, please write again.

Thank you for writing!