What Could Be Causing Severe Pain When Pressing My Sternum?

hi, I am 35 years old female. I recently noticed my sternum hurts BAD when I press down on it, I thought it was breast cancer but I had a mammo and ultrasound and they said no breast cancer. I don t have a fever or shortness of breath. no head aches. what could it be? no numbness any where either. but when I press down on the sternum the pain is a 10. not felling any lumps in breast on around ribs