Trying not to panic

After TTC #3 for 15 months, I got my BFP on Monday (2 different tests) after getting BFN on Saturday. AF was due Tuesday. Because my progesterone had been low I did blood test on Wednesday, HCG was 37, which I was told was low but I was barely 4 weeks. Repeated blood work today and was told my HCG went down to 25. So the doctor is thinking it's a false positive. I was told they will call me Monday to schedule an appointment and more blood work. Trying to stay calm until then but have any of you ladies experienced this and if so, what was the outcome?