Should I be worried

I found out I was pregnant way before I even had a missed period I took three home test they showed up positive so I was still in denial so I went to an er and they did blood work they said I was 4wks pregnant but when they did an ultrasound they said they don't see anything there and that my hcg was very low so they got to do another test on me to see where my hcg is right now should I be worried