Confused?? Please help

I am so confused by my last period I am 6wks but I had really bad cramps and went to the Er and the sac only measured about 2 weeks but my hcg levels were 2640 and that seems pretty high for only two weeks I am so confused as my daughter was a twin and I ended up losing her twin at 11 weeks could this be twins again they only saw 1 sac i don't know Advice please??????