Normal bleeding or probable miscarriage?

I am now 5 weeks pregnant. For the past week, I have had a pretty normal pregnancy, except for the lgiht bleeding that occured last week. I went to the ER on Thursday and since I was only 4 weeks and 3 days, the ultrasound didn't show anything relating to the baby. My HCG level was 765. The next day, I had another quantative hcg test done, and my levels were over the 1500's. I was happy and so was the doctor, but he stressed that I wasn't out of the wood quite yet. I went in today (Monday) and had one more test done. My levels were well into the mid 4000's, and my doctor was pleased, and then I had a big problem happen. I started bleeding. It was not a bright red, but more of a red with a brownish tinge to it. It only happened for maybe 20 minutes or so, and it wasn't like I was bleeding heavily, just whenever I wiped, that blood stuff was there. So then I got scared and called the doctor back. She wants another HCG blood test done on Wednesday, and then she wants me to go in for an ultrasound next week when I am 6 weeks, to make sure there is a heart beat and that everything looks normal, if this turns out not to be a miscarriage. I wasn't having any cramping at all, so the doctor told me that was a good sign. So any ladies out there have this occur to you in the very beginning of your pregnancy, and had a normal pregnancy after the first trimester! Please, words of wisdom!! Thanks ladies!