Hi there. Thank you for posting your question with Healthcaremagic. I will do my best to help you.
You are describing a haematoma
-- an organised collection of blood within the tissues of your buttocks. It will gradually disappear over the next ten days. You may also note some discoloration as the clot dissolves.
I hope that I have answered your questions fully and been of some help. If you have anymore questions please contact me again anytime.
Dr Andrew Rynne.