Suggest Remedy For Painful Bumps On Knee

My daughter had ACL surgery last March (2013) from a soccer injury. She just started playing again within the last month, over the past week she has developed a bump on the knee and its painful to touch and most recently the pain has gotten worse, even to walk on? we are making an appt. but wondering what that could possibly be or why this happened ? She did go back playing hard as she feel she has something to reprove herself, but did not hurt herself during a game or practice?