My 17 Year Old Daughter Came To Me With Major

My 17 year old daughter came to me with major pain in the heart area the other. I took her to the ER and they did a chest x-ray. They sent it off to be read and it came back showing her heart was slightly enlarged. This all took place on Wednesday. On Saturday, she started to drink a pop and it immediately started hurting worse than Wednesday, and I thought Wednesday was bad. What causes this and should we be alarmed by this? She is a very active and athletic girl.