[theBCnut]

Where exactly is the lump? Is it right by the anus, in a 4 or 8 o’clock position? Then it might be an anal gland issue. If it’s elsewhere, then it’s something else. Either way, just because it seems to come and go, I’d have the vet look at it. There are a few things I can think of that it could be, but you really should find out for sure.