What Causes Brownish Discharge During Menstruation?

i was suppose to get my period 1 week ago. I only had brown discharge instead. I am sexually active but we are super careful and we cannot recall any time to be worried about, but i havent had my period. if it makes a difference I have only been regular for about 3 or 4 months before then my period would come in 5 weeks, 6, weeks, 2 months, whenever it wanted to. i am worried i am pregnant?