Spotting with a positive opk?

I’ve been using Clearblue digital ovulation tests and for the past three days I’ve gotten the blinking smiley face. The weird thing is, on the third day (today) I’m experiencing somewhat heavy brown spotting. I don’t know what that could be. I’m 5 days late for my period, but I’m also pretty irregular. This also doesn’t look anything like period blood. It can’t be implantation bleeding either, because of the positive opks only starting three days ago... any thoughts? I’ve been trying very hard to conceive for about a year, and this is a little worrying for me. I’d appreciate any input:)