Sunburn definitely qualifies as an injury though I'm not sure if it'd be serious enough to cause this petechiae that you are talking about. I guess it depends on how serious the sunburn was. Did you get burned pretty bad? That said, your problem is probably the sunburn. Get some aloe vera with lidocaine in it and smooth that on wherever it itches or hurts. It can't hurt anyth9ing and should help.
Oh and I am not a doctor so if your symptoms persist or are extremely bothersome, you should see a doctor.