Can you feel anything on your buttocks? Have you looked in the mirror to see if there is anything there like maybe an abscess?
Please describe what you are feeling....
You most likely have an abscess starting deep within the dermis layer of skin/tissue.
Sebaceous cysts form this way and sometimes they have to be drained. You can apply some warm/moist cloths to the area and see if this starts to bring it to the surface. If it does, you can see if it ruptures and goes away on it's own. You must keep it clean at this point to avoid infection, though. If it doesn't go away on it's own, you will need to have it incised and drained. Most times, situations like this will involve antibiotic therapy as well.
If you notice any new symptoms such as numbness or a sudden rapid increase in size, you will need to go ahead and have your Dr take a look.
Hope this helps!
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