What Causes Irregular Periods With Brown Discharge?

i am 42 years old and i have a brown discharge for about 13 days now. I got my last period on 11 April and discharge starded on 28 April slitly and it was like periods for 1,2 and 3 May but still i have alittle. Why is this? I am 63 lbs waight and 4'11 hight.