What Causes Spotting Before Periods?

I started spotting 2 days before my period was to start on september 15 2010. It was pinkish on the 15th, the next day i had pinkish in the morning then by night it was brown. Since then I have been haveing brownish/blackish spotting. Not enough to need a pad or tampon but enought to have to wear a panty liner and change it 2 times a day. Then last night september 22, I went to the bathroom and wiped and say a hint of red so i put in a tampon thinking i finally started a normal period. About 6 hours later i took it out and half the tampon was redish brown and the other brownish black and not very full at all. Today i have had just brownish again. What could this be?