Why am I so "quiet"?

People I work with and go to school with want to know why I am so quiet. I don't talk much and when I do I guess I don't show very much personality. My family and close friends know I'm not always quiet, and I have a lot of crazy personality to offer, however Its just hard to show off when I am not comfortable with someone. Anyways when I am at work or even school I day dream a lot. I can think of so may scenarios all day long and think about many things that "I'm going to do!" but are always just thoughts that never get shared with anyone.Recently even my sister has told me that I may come off as "a stick in the mud" when in public, it was crazy to hear that because I don't feel that way at all. Its something I want to work on, but will take time and me being really open which will be really difficult. Its crazy how introverted I have become the older I get.