I know that sometimes after sex for me, I get an aching feeling in that region. I have researched a bit and found a few explanations. Deep penetration is the first and foremost. That can "bruise" the cervix and cause pain. It can also end up putting pressure on your ovaries and that can cause pain as will.
From experience though, if it doesn't go away in a few days or gets worse, go to the doctor asap. If it happens again try taking an advil to reduce any inflammation. Also go on some form of birth control. Pregnancy scares are well scary and can create a lot of stress on your body which can further throw your system out of whack. Try going onto the planned parenthood website. I think they even have some info about calling a nurse or getting a reply from a doctor. Good luck and be safe.