I fell on my back and hit my head and back. i had a lump on my head but it's gone. the lump on my back is still there. when will it go away?

Depends on the size: The lumps you have are either areas of tissue swelling which clear within days or pockets where blood has bled into a skin pocket called a hematoma. He aromas can take up to weeks to clearance and in some cases heal by organizing and don't fully clear but this is the rarer situation . Most of the time the blood is absorbed and over days to weeks clear completely.