I feel worried

Selena
Hello everyone, 
I'm 6 weeks & 2 days pregnant and Yestesday I went to the doctor to get an ultrsound but the was nothing but a sac seen is that normal? and they told me I'm 4 week half /: I'm so confuse and scared because it's my second  and 2 years ago, I was ectopic pregnant with my first 
Should I worry about that or not ?