My breasts hurt. Any idea why?

Sometimes that happens in the general vicinity of your period, or while you’re growing. If it keeps happening but only during your period, it’s probably OK but is worth mentioning at your next doctor’s appointment. If it happens nowhere near your period, unpredictably, or intermittently, and keeps happening, you may want to see a doctor specifically about that. 
Also, you may want to Google medical questions. This is just what I know off the top of my head in the general “maintenance of squishy flesh body” category.