What Causes Soreness In The Breast?

I am 24 years old and my last period started on Nov 6th 2010, next period is due on Dec 3 2010. My breasts have been sore since the begining of my last period 16 days now. They never get sore during ovulaion , the only time they are sore is 5 days to a week befor my period starts. Iam just wondering if this is an early sign of pregnancy or not. Thank you for your help