Wot?(Edited 18 seconds later.)
Capsular Contracture
Breast Implants that become hard usually indicate Capsular Contracture or scar tissue around the implants.
Warning the images on this site are not so nice for view
http://www.sowdermd.com/blog/breast-implants-hard-rocks/
They don't get hard during sex because THEY END UP HARD and even HARD as Rocks after a boob job - Not always but usually -