How sensitive are a girl's ? i accidentally hurt her?

im 16 and my friend is 17, she is really cute and thin with really big s like DDs. we started flirting alot and then we were fooling around. she put my hands on her and started to kiss me. i started grabbing and squeezing her s but after a few seconds she squealed OWWWWWWWWWWWWW and started crying. i asked her what was wrong and she said her hurt really bad. we had to stop and she was holding her s for about an hour afterward. Are really sensitive?