I am having pain, sort of like cramps after an orgasm. What may be causing this?

Without more information than you provided nobody is able to give you proper advice on this.
Where precisely does it hurt? What type of hurt, frequency and duration? How long was there actual penetration, how long did your orgasm take, and was that a single or a multiple event?
I strongly suggest you see your gynaecologist for a proper and safe dealing with this problem.
;)