Lines getting lighter- freaking out!

Took a pregnancy test last Wednesday, the day after my period should have started. It was positive! Confirmed with Clearblue digital and first response. CBD said pregnant 1-2 weeks along. 
​This whole week I've been having nausea and slight cramping- no spotting or pain.
​Took a pregnancy test this morning with First Response and my line is a LOT lighter. So I proceeded to freak out. Took another Clear Blue digital- that's still saying Pregnant- 1-2 weeks. I would think by now that my levels would go up and the lines would be darker! 
​Called the doctor- I'm getting a blood test tomorrow and Friday and they moved up my appt to next Monday to look at the results. 
​Just wondering if this has happened to anyone else and their pregnancy was fine? I'm so nervous I'm loosing our baby! ?