Being on the pill, people can get occasional changes to normal routine perhaps due to other hormone or thyroid imbalance problems. As a one off occurrence I would not be too concerned at the moment.
The cramping is causing spasms of the uterus which is losing the layer of old blood left from your last period. As long as there is no odor then this is unlikely to be an infection.
If the pain is very severe then you will need to see your doctor to rule out an ovarian cyst or possibly an ectopic pregnancy, although this is unlikely if you have taken your pill as directed. In the meantime use heat pads and ibuprofen tablets 400mg three times daily.
If this answered your question please click ACCEPT, use reply if there is something else you need to know regarding your question.Positive feedback or bonus is very much appreciated.