What Causes A Brown Discharge During Menstruation?

My menstruation cycle is 40 days. And I have sex only like once a week. Anyway, my menses should be coming today. But since last week, I have being seeing dark brown discharge on my pad. and today it is still the same. But when i wipe with a tissue on my vagina area, there is a abit of red blood on it. Is this a sign that my menses is coming or is it implamantation bleeding? Im worried. Pls help.