I Have A Lump In Between Mybutt Checks I Noticed

I have a lump in between mybutt checks I noticed it a month or so ago and then it kind of went away but now it is back I am kind of freaked out right now about anythi,g skin related as I have a bunch of moles and my mom and her sister were just diagnosed with melanoma. I have a dermatologist worth is Thursday for my yearlycheck up but this has me baffled...it seems to swell up around the time of my period and then go away...