Miscarriage or post-coital bleeding?

my husband and i had sex last night, and after when i went to the bathroom noticed some slight bleeding. In the morning i woke up to even more blood. I called my dr and went in. he did an ultrasound and said that he could still see the "tissue" formation in my uterus, and that its still to early on in my pregnancy to see much more than that. (im only 5 weeks today) but the bleeding has gotten worse since this morning, and the cramping is more as well. The bleeding is less when im laying down, and when i stand up, i feel a gush (heavy bleeding) the dr did a hcg quantitative test, told me to take a week off work and come back between 2 and 7 days for another hcg quantitative test.... but i just cant help but worry that i am having a miscarriage... any advice??