Mid-cycle bleeding when ttc?

My husband and I dtd last night and when I got up this morning I noticed some dark drown dried blood on my nightshirt and a little on my inside thighs. When I used the bathroom a little later I had blood dripping into the bowl (sorry if tmi!). Im on cd 17, I took an opk yesterday early afternoon and it was negative and my fertile week according to Glow is supposed to start today. Anyone know what this may be? I can't be pregnant and my period isn't due for a couple of weeks. Wondering if I should be worried about this.