slow rising hcg level

I am having issues with my hcg levels raising every 2 days. I'm also concerned that maybe I'm not as far along in my pregnancy as they think i am . I'm currently at 901 2 days ago I was 681 two days before 406 1 days before 260 two days before 106 two days before that 48.. I explained to the doc my last period was 11/ 17 and found out i was pregnant 11/ 23 so they r saying it should be the period before that which was 10/20 have done several vaginal ultrasound but no sac has shown yet due to low hcg level have checked my tubes for ectopic also but they are normal . Help I honestly think may be i ovulated early and I am 4 weeks instead of 7 .HELP SOMEONE