HI Had A Red Bumb On My Buttock About Two/three

HI had a red bumb on my buttock about two/three weeks ago and it came back the other day but the bump is bigger and it's painful to wear a thong n bowel movement. My stomach has been hurting like something awful the past two days since it poped back up. I got an std check last week so i no its not that. I also have a light brown muckas ball come out when i have bowel movement. Can u possibly give me some idea what it is or if it may be serious. Thank you