BFP then bleeding.

Last night I took a FRER and got a clear as day BFP (not as dark as control but could see the line very easily). AF was due that day. A few days prior I had also been testing and my tests definitely got darker. The FRER only just picked up the pregnancy last Friday (I got faint positives on one step & supermarket own tests up until that) so obviously my HCG was rising. This morning I woke up cramping (not that bad) and started bleeding. I had two bursts of actual blood but other than that it’s been light and brown. I’m assuming I had a chemical? Just confused as to how I got a clear positive hours before as I’ve only read about the lines fading and then you bleed. Not them getting darker.