What Causes Delayed Periods After Stopping Breast Feeding?

I have an 8 month old son who I nursed until he was 6 weeks, and then exclusively pumped breast milk. My period returned when he was 4 months old, and had a regular period the past 3 months consecutively. I pumped 2 times a day....going down to just once a day a month ago, and then every other day 3 weeks ago, and stopped pumping all together 2 weeks ago. I am now 1 week late for my period. Would stopping pumping cause my period to delay??? or should I take a pregnancy test?