Is it normal?

This is my first pregnancy. I went to the doctors on Friday because I had a history of low progesterone, and was having some brown discharge. He did an ultra sound and couldn't find anything.. is this normal? I'm only 4 weeks. He did blood work, and I have to go again Monday so results won't even be untill Tuesday..

I'm not having anymore discharge, or cramps but I'm a little worried that he couldn't find anything :/ I'm not trying to stress out , but after 2 years of ttc I can't help but be scared.