I'm sure there are exceptions, but one thing I remember reading several times is that breast cancer does not hurt. So, assuming that is the general rule, that fact, and youth, should keep the OP from worrying too much. Still, better to ease her mind!
I find mammos much less unpleasant if I schedule them for the first half of my cycle, when my breasts aren't sore. However, going through the "fun" of peri-menopause, they are now sore at times they shouldn't be, so there's really no good time to schedule a mammo. For me the pain is worst right along the top, where the plate digs into my pectoral muscles. Ouchie. But still, over pretty quickly.
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow