Has your doctor checked you for an ectopic? Like pp said it is possible to have bleeding through out pregnancy. I had a normal period in my first month with my son. But he is a perfect healthy 6yr old. How far along are you? It is hard to see anything on ultrasound, especially if you have a tipped uterus, until you are around 6-8 weeks. Did they see a sac or anything on the u/s? Good luck hun! I hope everything works out for you and you have a healthy pregnancy.
Very rarely does this happen, but it is possible to bleed throughout pregnancy. A friend of mine had bleeding throughout both her pregnancies and she had perfectly healthy babies! But if your concerned about it, you could alaway go to your local ER. Dr's don't really know what causes it, but it can happen... Congrats on your BFP