I would really like to know what's going on 'down there'?

First I'd like to apologize for the gross details but this is really starting to bug me. I'm a 20 yr old female and I have a fairly regular 40 day cycle. My period was due Sunday the 24th, I thought it was going to come because I started to get a light brown discharge that sometimes happens when my period is about to start, well it never came. Instead the last 4 or 5 days the discharge has gotten very dark brown almost black and much thicker and more copious to where I have to wear a pad even though it's not my period. I'm going to be really mad if I have this and then a period afterwards. I know light brown discharge is normal before and after your period, but 5 days of very dark brown discharge doesn't seem normal to me. I'm not having any cramps like my period would do, also it smells pretty foul. I'd like to know what's happening so I can maybe get it to stop because it's really annoying!