Hi, Welcome to HealthcareMagic Forum.
It happens if you have a Hormonal
Imbalance or if you have Pelvic Inflammatory Disease
I would like to know if you have,
1.) Abdominal cramps
2.) Severe bleeding
You should be getting yourself examined by a Gynecologist so that she may examine you and rule out the exact cause. If your Doctor doesn't find anything serious, then there should be nothing for you to be worried about.
See your Doctor immediately if this repeats for the second time.
Wish you Good Health.