5 weeks, please help!

So according to my period I was like 7 weeks pregnant, I went to the doctor today and looking at the ultrasound there was just a sack and nothing else was visible. My doctor told me it could either mean that I'm actually like 5 weeks due to the fact that I might have ovulated late or it could mean that I will have a miscarriage. I'm really worried. They had me do blood work to see what the pregnancy levels are and stuff, so we will see I guess. Has anyone been in this situation? And how did it turn out?