From my own experience, it's most likely a cyst, either on your ovaries or on your uterus.
Last year I had my uterus removed because I had a cyst that caused uncontrollable bleeding and I was HIGHLY anemic (had 3 iron infutions - they assumed it was CF related at first). The cyst was in a place that it alone couldn't be removed, so the entire uterus was taken. We left the ovaries, since I was having no trouble with those and we didn't want to cause more troubles than I already had to deal with, having CF (and not having to deal with the hormones issue.)
Since, I've had cysts on my ovaries. As explained to me, they will show up every cycle, and usually switch sides each month. I had one rupture and it caused so much pain that I had to go to emergency (I was vomiting from the pain). They gave me a shot of morphine.
It's something you should have your ob/gyn keep and eye on. Mine told me that if it happens too often, then they will operate, but as long as it's tolerable, they leave it alone.
I have sharp pains from time to time each month (that's when they are rupturing), but nothing like that one time.
Also, my ob/gyn said that she's experience that CF women have trouble earlier than non-CF (early menopause, and other female problems). And from my experience, I found it to be true.
Anyway, you'll need to get an ultrasound and a pelvic exam during your period - that will tell them the size of your cyst and determine the severity.
Good luck, keep us posted!
Sally, 40/CF - Uterus taken at 39. (still have ovaries)