You should have your doctor check you for Costochondritis. This is usually associated with lupus. It is inflammation of the costal (rib )cartilage at the sternum, the usual symptoms are chest pain, usually on the left side, numbness or tingling in arm, shortness of breath or difficulty breathing, and rapid heartbeat during relaxation. I went to the doctor with all of these symptoms, including thickening of the tissue around my left breast near the middle of my chest. I didn't know whether to tell him I was having a heart attack, pneumonia, or breast cancer! But he tested me for all and concluded costochondritis.
Although I do not have lupus, I do have costochondritis. My mother-in-law, who has lupus, also has costochondritis. It seems to flare up with stress. I believe that I got it from trauma to the chest. Also, people who have had open heart surgery can get it. The usual treatment is with steriodal or non-steriodal anti-inflammatories. However, I do not trust prescription medicine, so I try to manage it with relaxation and bromelain, which is a natural anti-inflammatory derived from pineapple.
I hope that this helps you in some way. The strange thing is that I just put this condition on the suggestion board the other day. There is not much information about this out there. But, if you do have this, do not worry, it is pretty much harmless, just a nuisance at times.
Wishing you the best in health!