Breakthrough bleeding?

So I had terrible crampy periods before my doctor and I finally decided it was time to stop them using nuvaring. I've been on it now for 3 months with no problem, but I'm starting to bleed.

Due to a miscommunication I've been replacing the ring every 4 weeks instead of 3. could this be why I'm bleeding? it's fairly light with no pain but I do have a headache. This could easily be due to stress because college. I'm just really paranoid. It's about 2 weeks early/late for me to have my period, and before I would've changed the ring again.

I'll be calling my doctor soon to sort things out but until then any advice? how long does breakthrough bleeding normally last?