5w/6w hCG in range but not doubling

Hi! 33 y.o. FTM here.

I had an MMC in May this year. Baby should have measured 10w but was measuring only 7w3d.

I'm pregnant again now, and got my beta hCG tests at 5w+4d (24DPO) and 5w+6d (26 DPO). My hCG was 19,800 (which is high for 5w) and 24,900 respectively, so an increase of just about 25% in 48 hours. I can't figure out what this means. The hCG level itself is good but not increasing even at the lowest expected rate for successful pregnancies (>=35%).

I also got an ultrasound scan at 5w+6d to rule out an ectopic (I'd been seeing some brown blood + pain). The ultrasound confirmed that the pregnancy was in the right place and that the embryo was measuring right. In fact, there was also a fluttering (estimated heart rate of 94). After expecting the worst, we were happy that instead of an ectopic, we got positive news. But today, looking at the hCG levels, we are again losing hope and preparing for the worst.

I was hoping to hear from this group - has anyone experienced anything similar? I have an ultrasound in a week but it is hard to stay calm. Looking for any similar experiences that can help me prepare myself for all possible outcomes.