How long before hcg gone?

My doc told me 3-4 weeks but I passed the 4 week spot and still have a pretty decent line. I think I’m having a period (it’s a little weird), and I know for a fact I miscarried, but I keep having dreams that they missed a twin and having that line just doesn’t soothe my fears at all. I feel like such a bother contacting them so much this miscarriage is so different from my last one. How do I know when to contact? It’s not like anything is terribly wrong I just have this eerie sense of something not being right.