What is this fluid filled bubble in my lower back?

I am a 22 year old male in good health. 2 and a half weeks ago I fell mountain biking and had a heavy impact trauma on my lower back. My entire lower back bruised pretty bad and initially i was in a lot of pain and my whole lower back was swollen. The bruising is pretty much gone and the pain has almost gone away... show more I am a 22 year old male in good health. 2 and a half weeks ago I fell mountain biking and had a heavy impact trauma on my lower back. My entire lower back bruised pretty bad and initially i was in a lot of pain and my whole lower back was swollen. The bruising is pretty much gone and the pain has almost gone away but now I have a large bubble in my lower back about the size of my hand in diameter. It feels like its filled with fluid and is raised out about an inch off my back. I saw a Doctor and he didnt know hat it was. Ive done a ton of research and I cant find anything about a fluid filled swelling. its on my lower left back. sorry for the wording i dont really know how to describe it. anyone heard of this before?
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