I had an injection now at injection site it is hard swollen n painful why is this?

Possibilities: This could have traumatized the muscle in which it was injected with resultant swelling/localized bleeding. Application of moist heat/heating pad can increase local blood flow & help this subside. The area could also become inflamed/infected if the technique was poor. This would be evident from increasing redness or swelling and any pus discharged from the site. Contact your doc as needed.