I got my cycle twice in the same month. now a week after my last cycle, there is brownish discharge like old blood. should i be worried?

See a doctor: A normal menstrual cycle (from the first day of one period to the next period) lasts 21 to 35 days. It is normal to bleed for up to seven days. Bleeding in excess of seven days or bleeding in between the two periods is considered abnormal. At your age, and if this is the first time this is happening, i wouldn't be worried. But if it happens repeatedly, see a doctor to get checked. Neg preg test?