What you are describing is similar to what I had happen last year & I'm afraid it did not end well. I had several + HPT's with one negative in the middle (all different brands) then got a + BT. I think my result was about 24 - 4 days later it was 12. By the time I was at 12 I had spotting which turned into light bleeding (way less than usual AF) which was for 3 days.

I hope your result is different.... I have seen it happen.

Had you been charting? My level of 24 was at 13 DPO which is extremely low so I knew things weren't right even before the spotting started.

Best of luck for a rise in HCG.