Okay so my girlfriend is on the pill and she has had a little breakthrough bleeding... we have had sex without a condom on pretty much when she was having this breakthrough bleeding (wasn't at the time bleeding as such) This was about a week ago and she woke up today with a headache and a massive stomach cramp that she described as "i couldn't move"... Do you think it is possible for her realistically to get pregnant? 

EDIT: She has been on the pill for about 4 Months and never had this before