Congratulations on your pregnancy! How excited you must be! I'm sorry you already have something to worry about, but it sounds to me like your doctor is doing the right thing. With my second baby, I started bleeding almost as soon as I found out I was pregnant. Because of the bleeding, and because I had a history of a tubal pregnancy, my doctor did an ultrasound as early as possible (about 5-1/2 weeks.) There was no sight more beautiful than that tiny beating heart! I also had progesterone and beta-hcg levels done weekly throughout the first trimester. My hcg's were fine, which was reassuring, but my progesterone levels were initially low. I began using suppositories tid, which corrected the hormone deficiency, but the bleeding continued anyway. Despite testing, we never did find an explanation for it. It stopped at the end of the first trimester and was never a problem again.
The only other suggestion I can make is to inquire about early Rhogam if you are Rh negative.
Try to relax, although I know you won't. None of us do until we hold that baby for the first time, and then it's on to a whole new set of worries!
Good luck and good health!