Question ladies....

My period was due yesterday and I've had some extremely light spotting since last night but not my period at all. It's mostly brown/old blood. My cycles are usually about 35 days. I had an ultrasound done on day 13 which showed a follicle progressing well and that I should ovulate very soon after that. I've been working with fertility specialists for over a year. hpt's have all been negative. Not sure if anyone else has had this happen and they ended up with a BFP days after a missed period and a lot of negatives?? Any advice is welcome! TIA!