It's really difficult to say Emma. But my first thoughts aren't pregancy to be honest (I'm a nurse by the way!!!). It could be cervical erosion - where the lining of the cervix erodes especially during or after intercourse. It's harmless but annoying. Or you could have something gynaecological going on which requires investigation. Is there any chance you could be carrying a sexually transmitted infection? Some of these can cause similar symptoms to what you describe (sorry I'm not trying to panic you I just need to cover all the possibilities). You say your partner has had a vasectomy so the chances of pregnancy are slim however, it's not unheard of so if I were you I'd invest �10 in a DIY test which are pretty accurate. This would at least give you a starting point as to what further investigations you may need. The contraceptive pill can also cause erratic bleeding and it also increases the likelyhood of cervical erosion. Do you take this for any reason other than as a contraceptive? If so, have you changed the type of pill recently? Is your partner particularly well endowed? This is another reason for cervical erosion. If you are losing alot of blood this may account for your dizziness/headaches as you may be anaemic. The stretching feeling could just be groin strain due to frequent sex (it does happen!!!!). Also, you don't say how old you are?
So in short, it could be a number of things. If I were you, I'd take a pregnancy test and go from there. If it is positive, see your GP. If it is negative either make an appointment with your GP or your practice nurse. The practice nurse will be able to examine you (she can tell by looking inside you if you have signs of erosion) and she can also carry out a smear to check for other reasons. However, as I said earlier, pregnancy is always possible. I never say never........
However, it's vital you get this checked. So please don't delay! Let us know how you get on!