Queen of cysts here!
(although that means absolutely nothing to anyone since every womans internal organs are different!) Since you've been on the pill I would tend to lean away from it being a cyst. My cysts are always on my left or right side so with yours being in the center, well, it could have been gas. I know I've had some doosey cramps in my day. It'll be like someone just stabbed me in the gut and it lasts only a few moments, but it feels like forever, and then it goes away.
Are you sure you ovulated already?? Are you temping? If so, then you would know if you had ovulated already.
I always have sharp pains on my girly insides so that isn't unusual for me and I guess if it happens again I would call your doctor.
BTW, I always have to tell women if you are getting an interal ultrasound: It should NOT
hurt when they do it, uncomfortable yes, but not hurt you. If it does hurt then say something right away!! I've found that for me, many of the women doctors do them quite rough and the men always seem to be gentler. Remember, just say something. I didn't say anything at first and I suffered quite a bit, so that's why I am telling you and anyone else reading this to speak up. (no offense to the women U/S techs out there who are gentle - I thank you
I hope it stays away and you feel better very soon.