What Can Cause Irregularity In Periods?

I have been using Nuvaring for almost a year now, and it has made my menstrual cycles last 3-4 days. This last month, the day before my period was due, my husband and I had unprotected sex. The next day it seemed as if my period was normal. However, it only lasted one day, which is not normal for me. Could I be pregnant? Is it too early to tell? And,most importantly for me, should I avoid putting Nuvaring back in until I find out?