What Causes Delay In Period After Breastfeeding?

Good day doctors I gave birth on 10 March 2010, breastfed exclusively for 6 months and after that breastfed 2 times a day until December. I had my first period in November and 28 days later my second one in December, but now I am still waiting. I cant be pregnant, we are very carefull. Can it be that my period is just not back to normal yet? Thank you, Lisa