Tried to skip period with NuvaRing, instead I've been bleeding for 16 days! What's going on?

I've been on the NuvaRing for 4 mos, so I thought I would try skipping my period. It's day 16 now and I'm still bleeding (not particularly heavy, but enough to be annoying) and having cramps. I read about breakthrough bleeding on your first try to skip a period, but I haven't seen anything that explains the cramping. Anyone know what might be going on?