Because you are referring to a very sensitive part of the anatomy,you don't have to be doing anything "wrong" to cause these kinds of marks. If you were engaging in "normal sex" as you say, getting caught in the moment, and being more energetic than usual would have easily caused this, and you wouldnt have noticed till afterward. I have had this happen before! The only precautions that you want to take is to watch for signs of swelling, redness that lasts for more than a week, or any signs of serious discoloration. If you notice any of these, for a prolonged period, see you healthcare professional.
