if there is nothing on the surface and no heat on the surface it is not an infection. just a bad injection you may have hit a vein or nerve. it can make a knot as big as a softball on your ass for 2 weeks ive had i happen several times. it hurts and its scary but it will go away. use a heating pad and massage it while you heat it. if you don't have one take a towel wet it, roll it up, put it in the microwave and massage the area with it a few times a day. it will go down. if there is heat at the surface you need to see a doc as it is infected. but if not its fine though it could bug you for 2 weeks in my experience. all a doc can do for a non infection is give you anti inflammatories not worth it imo to have to explain it all to him when you can use heat and massage