I'm 20 and have noticed that over the past few months my breasts seem like they are getting smaller. I have not lost or gained any weight and my diet hasn't changed. I would think that my clothes are just getting stretched out from being worn so much, but when I try on a new bra or shirt they don't fit the way they would have last year. Is this normal or should I be worried?
There is nothing to be worried about. Your breasts will continue to change size or shape throughout your life. Just wait until you get pregnant! Although you say you haven't lost any weight, it's quite possible that the underlying muscle that supports your breast, called the pectoral muscle, may have gotten stronger, replacing some fat. This could change the size and appearance of your breasts.
I would suggest that you carefully measure your breasts (put the tape measure around your chest passing over the nipple area), write down the measurement, then remeasure in a couple of months to see if there really is any change.