I am 29 and having pains in my left breast, its pain underneath and on the side and when i take a deep breath. it's not from my menstrual cycle i know?

Costochondritis: The fact that you describe it under the breast and worse with breathing suggests the problem is in your chest wall and not your breast. Most likely a condition known as costochondritis. A painful condition of what we think is inflammation and irritation in the muscles of the chest wall. Don't really know what causes it. Anti-inflammatory (ibuprofen, naproxen) moist heat to area can help.