Is Skipping Periods While Being On Nuvaring Normal?

I'm a 29 year old female currently on NuvaRing. I've been taking it as directed for almost 5 months now. I've been told to have the ring in for 3 weeks and then take it out for 1 week and that would be the time I'd be expected to be on my menstrual cycle. Last month I missed my menstrual cycle all together. This month I'm currently on the 1 week ring free week and there are no signs of menstrual cycle. I was wondering if skipping periods will on NuvaRing is normal? I've gone ahead and taken several pregnancy tests to verify that I am still being protected and they have all come out negative. My worry is that I'm no longer getting a monthly cycle. Thanks, Crystal