Brown discharge before period???

Hi all,
I hope someone can help me. Basically i have irregular periods (around every 3 months) my doctor said it was northing to worry about BUT since then (for the last 2 years) before my period is due i get this thick dark brown horrible discharge. It comes about 4-5 days before a period and lasts untill my period starts. I am not on the pill or any injections etc... and i am not pregnant. It probably started after the birth of my youngest child. This is how i know my period is coming, when i get this awfull stuff. Does anyone know what it COULD be?
Thanks xxxxxxxxxxxxxx