My 2 year old has been limping for 2 wks. blood test and x-ray show nothing. what could it be?

Musculoskeletal: The next possibility is a musculoskeletal problem. This could include a muscle strain, a short leg or a congenital problem with the feet (e.g., pes planus). Problems with the peripheral nervous system should also be considered, particularly if your child is experiencing foot drop. Next steps might include a consult with an orthopedist, a pediatric osteopathic physician and/or a neurologist.

Toxic synovitis: The most concerning thing would be an infection in the joint and/or bone. Trauma (a subtle fracture) also needs to be ruled out as it seems your doctor has. Toxic sinovitis is a swelling of a joint (usually hip) that occurs a week or two after a viral infection. It should respond to rest and Ibuprofen every 6 hrs within a few days. If it doesn't, or if there is fever and severe pain, go see ur dr.