Hcg level / ultrasound

So Friday I went for an ultrasound becuase I have been having pain on my right side.... I figured out my pain is round ligament pain. At my ultrasound I was 4.5-5 weeks but the doctor didn't see anything at all, not even a sac. But she did see fluid in my pelvic area and she thought I had a ruptured cyst. They did my hcg level and it was 650 Friday, and had me come in yesterday(Monday)  to do another hcg level and it was 2800. The doctor wants me to come in Thursday for another ultrasound. If my levels are normal why doesn't she want me to wait?