Ob-Gyn questions? Ask a gynecologist online.
I think you are not having with any problem in your breast tissue.
Yes chest muscles tenderness may be the cause or it may costochondritis.
But you should check whether it is present only left side of your chest or not,in that case heart problems are to be excluded.
If it only in midline gastric reflux has to be excluded.
It is probably chosto chondritis.
You should consult an orthopedic surgeon for examination and treatment.