You seem to have all investigations possible in search of a diagnosis for your symptom. Costochondritis is a clinical diagnosis and from your description it seems that you could be having costochondritis. Usually this resolves with pain killers. If it is particularly severe one could try a local injection of anaesthetic and steroid injection. This is usually done only if there is no suspicion of infection. Most of the time this stays for some time and settles. At times it may take several months. You do not need to worry as eventually it settles.
If your symptoms are worsening or there is any swelling coming up do visit an orthopaedic surgeon.