Dates not adding up - help

So when I went into the doctor for the first time - I thought I was about 6 to 7 weeks along. My last period was in Jan. However I got off the pill mid cycle in Feb. had some withdrawal bleeding but my period never came back. My doctor at the appointment said that according to my last period I should be 12 weeks but am only measuring 6 weeks - has anyone had anything happen to them like this!?!? Please help!! I go back in 2 weeks for another ultrasound- thanks