What Causes Brownish Spotting Before Menstruation?

I'm 27 about 9 stone 11. I am usually on my period for 5 days on time. This month I was 2 days late and only had brownish/pink spotting then nothing for the rest of that night. Next morning I had a bit more like it was my period more red and I needed to use 2 tampons that day. On the night nothing but a few brown spots on the pad. Nothing the next day or day after apart from bits of pinky brown when I wipe? Any ideas? I took pregnancy test and was negative. Thanks