Anybody want to help..?

I miscarried early Feb. - no period since. Had sex March 3 (the only time since miscarriage) and took a test March 21..positive. On the 23rd my hcg was 89...over doubled since my bloodwork on the 21st. No clue how far along I am without a period. Thoughts? 
Have an ultrasound Thurs and am nervous there won't be anything to see if I'm too early?