What Can Cause Delay In Periods After Taking Out Nuvaring?

Hi. I just have a question because I'm a little confused. I am on the nuvaring have been for about 5 months (had a baby in august of 10) and usually when I take my nuvaring out for a week I'll start my period for about 4 days and then I'll start it again the day after I put a new one in. I have did that ever since I got it, and now I didn't have a period at all when I took it out and I put my new one in 4 days ago and I still have not started my period. I always get the feeling like I'm going to and nothing happens.. is this normal or could I be pregnant?