Hcg levels

I recently found out I was pregnant and got my blood drawn on Monday and they were 600 and I am a early at 4-5 weeks. Today I went back two days later and they didnt double they rose to a little over 900 though. My doctor sounded concerned and I have to go back Friday to get more labs and ultrasound. Has anyone else had the same thing happen where their hcg levels didnt double??