I cant tell you if its an STD.
But I can tell you about two times when I was injured.
Once with my first wife. She could suck a golf ball thru a garden hose. Anyway, without being too graphic, she gave me a hernia. It was painfull as hell.
The second one was during my younger years. I dated a very... atheletic woman. She bounced around on me so hard that she actually severely brused my pelvic bone.
I doubt you have the latter, but the point is, its possible to get a physical injury during sex which is not related to STDs. Also, your testies could get twisted into a knot. But if feels you took a soft kick to the jimmy, you probably got a slight hernia from sex. Give her a gold star and dont worry too much about it.
You wont know anything for certain until you get the results, but you can probably relax some.