"An 18 year old female on Birth Control Pills (BCP’s) complains of LLE pain and swelling since this morning. During the physical exam, the patient suddenly feels SOB and has pain when breathing. What is the most likely cause of her new symptoms?"

The cause of her new symptom is Deep vein thrombosis, or DVT. which has traveled to her lungs cause Pulmonary embolism.  

Explanation:

Women who take birth control pills are the higher risk of developing DVT. Estrogen in conjunction with birth control pills increases the incidence of blood clot in leg (DVT) or may cause a clot in lung (pulmonary embolism). The clot through the leg has the ability to travel all the way to the lungs via circulatory system, and cause pulmonary embolism. Pulmonary embolism causes a blockage in arteries of lungs. This situation causes shortness of breath and chest in pain.