I am having a dull ache in my sternum (mid-center), not constant, and am worried what the cause might be. the underside of my breasts ache as well as the sides.

Costochondritis!: Rib/sternal cartilage inflammation is a common rheumatic disease. It is inflammatory, but causes, in and of itself, no morbidity, other than the discomfort and pain. See someone in rheumatology, after you see your pcp!

Not too concerned: Chest pain in your age group is usually due to muscle pain or stress related. Go see your doctor and suggest an ekg just to be sure it's not heart related. Acid reflux symptoms usually present with burning pain in center of the chest, aggravated by eating spicy foods and intake of caffeinated and alcohol drinks. Quit smoking if you do smoke.