Advise?

I went to the Dr. For pain in my right side the other day the Dr. Decided to do a early ultrasound and didn't see anything so she sent me to do my hcg levels they were 250 I have to go again Monday to redo my levels to make sure they are doubling. The thing is I don't remember when my LMP was I only started getting positives on hpt since Oct.12th could it be that I am just to early that's why they didn't see anything?