Spotting

Laura
I'm currently on day 17 and I've had light spotting (brown in colour) on/off for the last couple of days.
All OPKs have been negative, my flow chart says I'm on day 3 of my fertile week. BD yesterday and the day before last. Anyone know what this could be?
I am having trouble with OPKs been using them for the last 3 months and have never got a convincing positive 😔