What Causes Irregular Periods?

My son is about 11 years old now. I am also the type of person who has normal monthly menstruation and cannot miss a month. However, recently I have divorced my husband and am living with someone else. There has been a change in my menstrual cycle allowing me miss months. This has led me to think that I am missing months and getting pregnant again but then the second month, my flow would be heavy. What is wrong with my cycle or rather was I ever experiencing pregnancy?