Yeah, I agree go to the doctor right away. It could be a number of things. It could be a early sign of miscarriage, ectopic pregnancy or molar pregnancy. On the other hand in the first trimester it could be simply be implantation bleeding, irritation from sexual intercourse (cervix sensitivity) or an infection in the pelvic (like a really bad urinary tract infection). Or something else your doctor can ease your mind with - it may be something that can be helped with medication? I don't know. I have also heard some women do spot through out their pregnancies...I didn't - I would go ASAP.
Bleeding During Pregnancy : American Pregnancy Association (http://www.americanpregnancy.org/pregnancycomplications/bleedingduringpreg.html)
Take your Materna Vitamins (Folic Acid now if you haven't already) to assist in fetal development. Please go to the doctor so he/she can tell you what is really going on.