Rachel I definitely commend you for being so proactive at your age! I think many young women take breast cancer for granted, thinking it's an "old lady" disease when in fact that is NOT true. I was just 31 my first time, and cancer does not run in my family.
I think what you are experience is just normal breast changes for a growing teen. Your breast will go through many changes prior to becoming an adult. Inverted nipples are quite common, and one of the many reason women can't breast feed. I had them as well as my mom, and as you said you know one breast can be slightly larger than the other.
But like PJ suggest, seeing your doctor may be the best thing to ease your mind and rule out risks.
Best of luck to you!
Angi