Miscarriage?

Hi, I’m 4w4d and I had some brown spotting yesterday so I had a trans abdominal sonogram which showed nothing (not even a sac) and had blood drawn. My hcg is 40.1 and progesterone is 2.89 which I know is ridiculously low. They want me to come in to test my blood again tomorrow to see if my hcg is going up but from everything I’ve read, a progesterone level that low is almost certain to end in miscarriage so I’m not sure why they even want me to come back for another blood test. Any ideas why? Has anyone been through anything similar to this? I had a mc a few years ago and then low progesterone in my pregnancy with my daughter (had to take supplements), so it’s obviously an ongoing issue for me.