Orgasms cause the uterus to contract, which for most women is safe, but for some not safe and can lead to bleeding and/or miscarriage. Hopefully everything is ok. If the bleeding was minimal with no recurrence, hopefully it was just a one-time thing. Check with your doctor / midwife and tell them about the orgasm. Good luck!
Its normal to have cramping at this stage as everything is growing and stretching.