Is it normal to spot with a NuvaRing?

I have been on the Nuvaring for three months now, and I am keeping it in for the four weeks then changing it because my doctor said there's no need for me to have a period. About two weeks ago I started having dark brown discharge and it has gotten more heavy, but only when I run or stand up real quick. Is this normal or should I call my doctor and find out what is going on?