You should see a doctor for this problem. What most probably has happened is that you have a large collection of blood in your thigh following the fall. This solidifies itself but then goes on to liquefy with time as the collection is gradually absorbed. However, if there is an external breach associated with it then there is a chance for the hematoma
to get infected and cause additional problems otherwise it should go away with conservative treatment. However, some surgeons prefer drainage
of a hematoma to expedite the healing process in large hematomas. In any event you should therefore see a doctor to find out if there are any complications arising out of it and if it needs additional treatment.