I fell down the stairs real hard in november last year. it hurt my left bum. now the bruises have gone but it left big hard oval lump around 10 cm long on my bum, it doesn't hurt, but how to get rid of it? and how long will it take?

Wait: Looks like you developed a collection of blood in the area you fell. Leave it alone if it doesn't cause any discomfort. It will take several weeks to resolve on its own

Lump on buttocks: May be a hematoma and that is still resorbing, or may have become cystic or fibrosed or bony- myositis ossificans or rarely may be a mass ( fibroma, hemangioma,neurofibroma that came to light after trauma . Would consider sonogram/ xray to identify the nature of the bump. Consider massage and warm soaks and time, sometimes excision is needed f/u/ w/ doc