I am 30yrs old mother of 2.still breastfeeding. I have been having this piercing pain in my chest on my left side. What could it be and what do I do?

Chest pain. Could be problems with your lungs or heart or chest wall muscles or bones . The accuracy of the diagnosis decreases in that order. Mostly due to infection, inflammation or diminished blood supply or trauma . Please seek medical care immediately for exam and diagnosis and treatment.