maybe you contracted an std or maybe you may be having an early miscarriage..i personally havent and dont know of anyone who has had that as a pregnancy symptom
i would still go on monday, but it isnt unusual to spot in pregnancy. my last 2 pregnancies i spotted after sex and i have a 12 mos old and am 36 weeks pregnant so i wouldnt worry about it. they said once it was from my cervix and the other pregnancy they didnt know but did tell not to have sex...