Does A Stabbing Pain At Bottom Of My Stomach After Sex Mean Pregnancy ?

Hi I hope you can help me, I came off my pill (cilest) on 26th March 2011 and got my period on the 30th March 2011. Me and my partner were trying to conceive (unfortunately he has now changed his mind) My period was due yesterday (27th April 2011) and has still not came, I done a home pregnancy test this morning which came back negative, I have been feeling a little nauseous with some cramps (although not as sore as when have my period). I have also had a white discharge for a good few days. The last time we had sex (24th & 25th April 2011) It was quite painful, i could feel him inside me like he was hitting off the bottom of my stomach (this has never happened before) Then after whatever way i moved was extremely painful like a stabbing pain at the very bottom of my stomach and also at the bottom of my back, after a few hours it went away. What could this be and could it be too early for me to tell I am pregnant? I don t want to go back on my pill until I have a period. I would be grateful if someone could get back to me on this matter ASAP. Kind Regards G x