What Causes Delayed Periods?

When my baby was around 6.5 months old I finally had my first period since his birth in November (I am nursing). It lasted from May 28th until June 2nd. From June 15th until June 18th I had another period. My husband was deployed on May 30th to Louisiana for the oil spill. I went to visit him from June 25th until July 6th. It is now July 16th and I do not have my period. Should I be concerned? Is there a chance I could be pregnant? We did not use any protection.