Mine was in the same situation as urs, except its lump was under its body. My sister and I never noticed it, cause Bijou was sooo active, always running up and down, and the lump was underneath it, so we never saw it, until we were were going for a holiday and had to pass her to my aunt. Bijou lived for only 2 more weeks I suggest that you leave your hamster be, cause surgery could cause some trauma to the poor hammie.