Before I Make A Trip To The Doctor's Office, I

Before I make a trip to the Doctor's office, I want some insight. 3 days ago I took notice in the shower a small pea sized soft squishy purple-y bubble on my left butt cheek where it "closes". It isn't bothersome nor painful. I applied hot compresses for 15 mins 3x that day. On day 2 bubble went down in size but the area around it looked like a bruise (it didn't on 1st day). Again no pain nor tender. Yet soaked my bottom half in the hot tub for 15 mins. Day 3- the skin that held the once filled bubble is stretchy much smaller in size than Day 2 & yet still looks like a purple-y bruise. Any ideas?!?