What Causes Delay In Menstruation Other Than Pregnancy?

my boyfriend and i had unprotected sex about 10 days ago. I am on the pill and have been for about 2 months. this is the week i am suppose to get my period but it is a little late. I just recently had a baby and prior to the pregnancy my period came around the 20th every month and post pregnancy it now comes significantly earlier. could i be pregnant or is it possible that my period is just adjusting back to what it's normal schedule was before? also, i've been feeling nauseous.