Low HCG levels! Help!

Casie

Per my last period (First day September 26) I should be almost 8 weeks pregnant.

OBGYN sent me for bloodwork and ultrasound same day. U/S showed thickening of uterus a cyst in my left ovary and no sack. HCG levels came back at 271

OBGYnlN said that it seemed like I wasnt as far along as they thought (they are guessing 5 weeks) so I am getting bloodwork done every 48hours. She wants to see my HCG levels doubled however

Thursday: 271

Monday: 520

Wednesday: 687

I know going up is a good sign but they are no where near doubling. I need help!!! And google isn't seeming to give me the right answers. I have pregnancy symptoms like nausea, slight cramps and peeing like mad. No bleeding.