What Causes Pain In Arm Of A Child?

my 4 year old has had a fever & incredible pain in his right arm for 2 days. We took him to his pediatrician & they did xrays on his arm but there was no sign of a fracture or a break. Any ideas? He has been on tylenol & ibuprofen at 4 and 6 hour increments respectivley. He seems better after the ibuprofen, but the pain comes back within a couple hours. He cannot get comfortable.