What Causes Vaginal Spotting Before Periods?

I recently had a smear test and it came back clear. Before the smear, I mentioned to the nurse that I had been spotting between periods (mid cycle) so she checked for infections which also came back clear. I was advised that the spotting was due to hormones/ovulation. However, recently the spotting has got slightly heavier/thicker and I have noticed a couple of small clots - there is no pain though. Also, I have been spotting a couple of days before my period actually comes, which has never happened before. I am not sexually active and have not been since my last smear test.