What Causes Delay In Periods When On Nuvaring?

I am on the nuva ring and I am pretty consistant with my schedule. I have been using nuvaring for 4 months after switching off of depo (only had one shot). I took my ring out on Monday night (1/23) and my period still hasnt came. Absolutely nothing. My boyfriend and I use condoms when we have sex, which isn't a lot, maybe once or twice a week. I have also taken 3 pregnancy tests which all came up negative. Should I be worried?