Hypochondria Support Group
Hypochondria refers to excessive worry about having a serious illness. Hypochondria is often characterized by fears that minor bodily symptoms may indicate a serious illness, constant self-examination and self-diagnosis, and a preoccupation with one's body. Many individuals with hypochondriasis express doubt and disbelief in the doctors' diagnosis.
Your symptoms sound very similar to a condition I have: costochondritis. It is caused by inflammation of the cartilage around the ribs/sternum. It often hurts on the left side of my sternum, under my left breast, and radiates through my left breast. Sometimes I feel the pain on my right side or all the way through my rib cage. If I press on different areas surrounding the pain it hurts and feels like I'm bruised. I can also feel the pain for a few seconds after I've touched the tender spot(s).
Talk to your GP. If it is costochondritis he/she would be able to diagnose it with a physical exam and asking you questions.
I hope this helps! Take care!