This recently happened with my 3yo niece, except she was complaining of severe pain in her legs and arms. She wouldn't walk and nothing would calm her. Initially they thought it was growing pains, or arthritis. She received bone scans, MRI's, and a biopsy on an abnormality found in the bone on her arm. Turns out it's Leukemia
She's is now on a path of 2.5 years of Chemo.