Hi!
This can happen for a variety of reasons.
Was this your first time? Or maybe after a long time of abstinence?
Do you have a period near?
Is your partner unusually large?
Was the "intimacy" a little rougher than usual?" Sometimes "Doggy-style" (you on your knees, he, from behind) can cause him to penetrate you very deeply and he can "bump" into your cervix. Painful and can cause some cramping afterwords.
Was there enough lubrication? Were you not ready and maybe didn't have enough foreplay?
Do you have pelvic inflammatory disease: an infection in your uterus or fallopian tubes. Very common and treatable.
Were your orgasms especially intense? This can sometimes cause muscle contractions, even of the uterus which would feel like cramping.
There is also a condition called vaginismus: Highly treatable! :)
http://www.vaginismus.com/faqs/vaginismus-questions/what-is-vaginismus
Do you have a yeast infection?
This happens to a lot of women, myself included. Most of the time it's just one of those things. However, just to reassure yourself or if this is a reoccurring thing, I would call your Gynecologist and ask, can't hurt! (no pun intended!)
