I think I felt movement?!

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I think I might have felt a baby movement the other night? It was only once, but it was nothing like anything I have ever felt. I think my weeks might be off. When I went to my first appointment I thought I was 9 weeks pregnant. However, the ultrasound only measured 7 weeks. Babies heartbeat was normal though. When I found out I was pregnant it was through a blood test where my HCG was measured to be 406. At that time I had calculated myself to be 4 weeks. With my new due date, that means at the time of having a 406 HCG level I was only two weeks?! Is this even possible at two weeks?? I’m a very slender girl so having a small baby wouldn’t be all that surprising.