Could I be pregnant? Please help.

I had unprotected sex during my fertile window-- glow says fertility 15% that date (wednesday March 4). We used the withdrawal method which is why I'm worried; it's not effective. 
My period isn't due for 15 days so it's not late yet, but I have a really irregular menstrual cycle due to ovarian cysts and anorexia nervosa. I've just been noticing ovarian pain and dizziness within the past day, but I'm not sure whether that can be attributed to pregnancy, ectopic pregnancy, another cyst, or just mittelschmertz.
I'm 17, so I'm really paranoid, but I need to know what my chances are.