Can't stop thinking! It's driving me a tad nuts :/

cassie
So like a few women in here have stated...according to my last af i should be 7 weeks tomorrow BUT i went on tues and they couldnt see anything on a u/s..my hcg levels in my blood were only at a 400..im scared and just cant stop thinking what could happen..any tips? I found out like 3 weeks ago on 4 pregnancy tests that were positive. :(