HCG levels aren't growing rapidly...

An
So I'm 5 weeks, 4 days.
I had blood work done Monday and Wednesday.
I believe my numbers were in the 300s on Monday and in the 500s on Wednesday.
I have a sono scheduled next Minday, but they want to see me tomorrow.
Has anyone else experienced this? I'm kinda nervous now and hope everything is okay. Kinda hard not to be worried though. I haven't had bleeding or anything.