Dear Friend, We understand your pain. You got an injection for muscle spasms in left upper thigh. I guess you mean your hip (gluteal region). I would like to know if you have developed some weakness or wasting of your leg? If yes, you might have developed injection neuropathy, which occurs as a result of I.M. injection injuring sciatic nerve. But the way you are saying that your pain is radiating to your leg, it seems to be neuropathic pain. Usually injection neuropathy has wasting and weakness of limb, not a radiating pain. Do you have any back pain? Most common Differentials of this kind of pain are Compression neuropathies, PIVD, Meralgia paraesthesia (however, pain doesnt radiates to leg). Sometimes, hemibody pains are - Thalamic Syndromes. You need a neurological examination to find the cause. I suggest you to meet a neurologist , get an X-ray LS spine let us know the result. Stay Healthy. Dr Anshul