Confusing blood test

Hi! So I've been TTC for three months now. Last month I had a early mc (5w2d). But we continued trying and as per my blood test at 3w4d I am pregnant! However. I have yet to get a BFP on a hpt. And my levels at my BT were 18 HCG. This was three days ago, currently 4w0d. I'm getting nervous that maybe my HCG levels aren't doubling like they should or maybe I'm having yet another MC. Anyone else have low HCG to start and it turned out okay?