Can the doctors be wrong?

So the last week of January I had a small bit of bleeding (nothing like a period). It lasted maybe 3 days and there was barely any blood. It all looked like old blood. About a month or so later I found out I was pregnant. I told my doctor about the bleeding and she just wrote it off as a period. We got a ultrasound done and he measured a little over 8 weeks. Now I am 27 weeks pregnant and measuring 5 weeks ahead. Could she have been wrong about how far along I actually am? Cause I feel like that's the case.