Opinions please?

I went for my first ultrasound today, she told me it was too early to see anything. The doctor called after getting my blood work back and receiving the pictures and told me my uterus measured at 8 weeks but there was no actually baby seen in the sonogram. She had done both a regular and vaginal ultrasound. He said to come back on Friday for more blood work to see if it is an on going pregnancy. I'm so scared and my boyfriend just is saying to relax but how do I do that? Why would I show pregnant and why would there be no blood if something was wrong ?