How is it that I have SEVEN cavities??!?

The last time I went to the dentist was in August 2010, and I was cavity free. It's been seven months and now I'm being told that I have seven! I brush my teeth 3-5 times a day, I floss, and use mouthwash. I have NEVER had a cavity before in my life. I don't understand. Could it be my diet? I've been a vegetarian for the past 14 months but I don't see how it would affect it? Also, my last dentist put in sealants.. apparently three of the cavities were growing/trapped inside of my teeth. And I've had the same tooth brush all year, but I recently changed it after my appointment. Are there any other outside factors besides poor brushing? I'm devastated! I'm afraid to chew!