Abnormal Period

On September 24 I got my period but it was very light and the blood went from red to brown within a few hours. I had my period for maybe a day and it went away after that. (It normally lasts 4 days.) I took a pregnancy test and it was negative. So this month rolls around and I'm waiting for my period to come which it did, today. Only thing is the blood is brown again and its not alot. I haven't taken a pregnancy test yet because I don't think I'm pregnant. What could be another reason for this?? Anyone else have this problem?