Spotting on cd 16

I'm 37, mom of 3 from a Previous marriage. I've had my period since I was 12. It's always been regular, 27-29 days apart lasting around 5 days. I've also been TTC the past 4 years with my husband who has no children of his own yet. I've been to my gyno and a specialist and all they've told me is since I have 3 already, it must be my husband. He's had testing and results were low count and motility. Last pap smear was at the end of May with normal results. Before my last period I had a dizzy, shaking spell, almost faint and racing heart. I went to the dr for blood work and it was all normal and since I have mitro valve prolapse, they did an echo on my heart which was also good.  They also did a pregnancy test which was negative. For the past year I've had brown spotting a couple of days before AF. Last month I had 3 days spotting followed by a short 3 day period. Period started July 24th. On CD 16, 17,18, & 19. I started having brown spotting , mostly when I wipe, some has been red. No spotting yesterday and now today red spotting again. Took a pregnancy test and was negative. Has anyone else experienced this and if so what was the cause? Google doesn't help much because they say ovulation spotting lasts only 1-2 days & implantation bleeding is closer to aunt Flo.