What Causes Sharp Pain In Lower Part Of Abdomen?

I am 18 5"9' and weigh 219lbs right now. I have had one child already and I am on the bc pill as of right now, although I have missed a couple recently. My problem is that I woke up last night with a sharp pain in the lower part of my abdomen and in my crotch. It felt like pressure and it made it worse when I sat to go to the bathroom. I had been constipated a little before that and earlier today I had managed to go to the restroom, now the pain is not as sharp but it is still there. I'm not sure to as what it could be?