YES BUT IT DOES NOT SWELL
Hi
Welcome to the MedHelp forum!
A lump on your buttock that comes and goes can be a cyst or it can be an abscess (localized collection of pus) or it could be a lipoma (localized collection of fat). The other possibility is that it is a mobile lump like a neurofibroma (localized collection of nerve and fibrous tissue). When the swelling appears consult your PCP. It is difficult to comment beyond this without examining.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!