Missed period and letrozole

Has any one missed a period not been pregnant but definitely ovulated? I had spotting which was barely there 3 days only when checking Cm and 2 days when I wiped it was brown in colour this happened 5 days before I was expecting AF 1.5 weeks after the spotting started- neg tests and blood tests still no AF. On and off cramping since the last day of spotting but very very mild. This is was our last round before ivf and of course something had to go wrong 😩