I had given birth dec 3rd 2014 and me and my fiance have been having sex when i was ovulating and now i have really bad cramps?

See your OB-GYN: Generally the advice after delivery is for "pelvic rest" until your post partum visit- 6 weeks after delivery. Not clear how you determined that you were ovulating, but there is a possibility of pregnancy even this soon after delivery (if you did not use contraception). Infections could also cause the symptoms you describe. Check with your OB-GYN to clarify your issues.