Woke up with pressure on the left side of chest when breathing, it's the 12 day of using birth control pills. why is this happening ?

Consult your MD: I encourage you to contact the physician who prescribed you the birth control pills and go in for a physical exam. Pain of cardiac origin in a young woman your age is unlikely. However, you should not smoke while taking BCP. If you have pain when you take a deep inhalation it could be pleuritic pain. Have an in person exam but it will likely go away on its own. Do you smoke?