Pelvic Pain

So it doesn't always happen, but sometimes after sex I will get period-like cramps. They hurt really bad for a little bit and then it's just a dull ache for about an hour. Sex itself doesn't hurt, just afterwards. Tonight I used a vibrator and I orgasmed and am now having that same pain. It's the first time it's happened on my own. Does anyone else have this issue or know what might be the cause of the pain?