2nd Hcg levels

So I got my first level drawn in Tueday which I though I was 4 weeks 4 days. My level was 915 my OB said it was low and that I needed to come back in for another lab draw. So on Thursday I went in and I should have been 4 weeks 6 days and my level more then doubled it was 2,318. My OB still is telling me it's low and that I must have conceived at a later date. However, everytime I look online it showed both levels were normal for that many weeks. On Wednesday when I found out my levels I took another digtal weeks estimator and it showed I was 2-3 weeks which would make me between 4-5 which matches with what I thought I was. 

Has anyone else had these numbers like me and was told they were low or that they conceived at a later date? My OB appointment is May 11th and I get my first ultrasound so now I'm curious about how far along I really am! Any ideas!??!?