What Causes Brownish Discharge During Menstruation?

i am 47 years old. Have had very regular periods every 28 to 29 days. However i missed my period this last month. I was suppose to come on the 28th JAn 2011. I have not seen it as yet. I took a home pregnancy test yesterday and it was negative. However i spotted brownish red and also pink during the last 9 days but not everyday and only for a short time. Whenever i had sex during this time of missed period i will spot pink soon after. The brown spotting comes with a smelly odor, but not the pink spotting. can you help me?