Why does my left thigh hurts so much after I injected into it?

Need more info. Pain after an injection can be from the medicine injected. Some cause a lot of inflammation and pain. Other times there is a bruise if a blood vessel was hit; that can cause soreness. These causes usually resolve quickly as the medicine/blood is absorbed. A worse cause is if a nerve has been hit. That can hurt for awhile. Such pain usually radiates into other areas.