HCG levels

I found out Monday I'm pregnant. My progesterone was 22 and my HCG was 59, indicating 1-2 week pregnancy. I went back Monday hoping my numbers doubled but they only went up too 100. The dr was fine with that. I went back today (Friday) and they were only 128.3. They want me to come back Monday because they weren't happy with the rise, they said they should have doubled. Has anyone ever seen numbers that low with a successful pregnancy? I know I'm really early, but nervous.