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The symptoms you have described of having lump/growth of size of tennis ball on your butt cheek is medically called as hematoma.
A hematoma is a localized collection of blood outside the blood vessels usually in liquid form within the tissue.
A hematoma usually is not a cause for concern.Apply gentle heat with warm towels, a hot water bottle, or a heating pad. The heat is applied for 20 minutes at a time to promote absorption and repair.Hematoma will resolve by itself within few weeks.
Despite how they feel, hematomas are not typically cause for concern.Hematoma will resolve by itself within few weeks.
So as it has been 2 weeks since your fall from stairs,this may take time of another few weeks.
So I advise you to just wait & watch and apply heat as described above to subside it more quickly.
Hope this helped
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