When I was pregnant with my DD, I had the same thing sort of happen to me. I don't think I'll ever forget those first beta numbers!
They were: 221 (great), 639 (great), 737 (oh crap!)
I was so devastated that day, and the nurse who called me (it was a Sunday) couldn't tell me much... she just said it could work out just like it could not work out, we just had to wait an see. A few days later, we were in the 1200 range and then 2100 range and all was good.
When I finally went in for my 6 week u/s and everything looked great, I asked my RE about the betas and he said, "They're just numbers..." I don't know if he truly meant that or knew he was able to say that because things were fine after all, but my point is that numbers do weird things sometimes. That's all I learned from my experience, but I wanted to share if only to reassure you that everything could still be fine.