Slow Rising HCG Levels?

Okay, I got a positive pregnancy test on 2-8-18. I took some more over the next few days and the line continued to get darker. Then on 2-13-18 I started my period. I called the dr and came in for an ultrasound, and it was too early to tell. They sent me for blood work to get a HCG workup. My quants were 25, then two days later 29. Then two days later 39, and then another two days now it’s 43. We did another ultrasound yesterday, and they don’t see anything still. My drs are saying it’s either chemical or ectopic, but they are confused with how my numbers are. Just wondering, has this happened to anyone else? I just feel like I’m some rare lab rat that this has happened to and they are at a loss.