Mid cycle spotting?

I have very regular cycles, 33 days, ovulate day 20. This month, over last two days, ive had pink/brown spotting. Not alot, a touch on tissue paper yesterday and none the rest of day and again this morning, none again since. Mid cycle spotting isn't normal for me and was just wondering if anyone knows why this can occur? I dont think I'm pregnant as its been an odd month, having used my opks, house move etc so myself and partner been a bit distracted and just the odd bd fitted in. Any suggestions?