I too had this hard area low down in my cleavage - it's where they chip into the breastbone to attach the blood vessels. Now more than thirteen months post DIEP this hardness has gone - I think some of the hardness may have been scar tissue.
I have a hard area in mycleavage after my reconstruction. i am not seeing my PS till next month but when I went for a check up with the surgeon who did the mastectomy he said my rib had popped out and this was not unusual! Anybody else had this happen and what they did have done to put it right?