What could it possibly be?

Well, to put it blankly: I've been having troubles with my heart. I'm a 16 year old female and in pretty good physical health. My only problem is I love to run, but I can only run for a few moments. My heart starts to hurt like crazy. The pain is indescribable. Sorry I cannot give you any more detail than that. Then a lot of the time when I go to take an extra deep breath, My heart skips a beat. So, I try to take that deep breath again and it keeps skipping a beat after quite a few tries to get that breath. I've had this problem for quite a while now, but I just now said something, because it has gotten worse. I haven't gone to the doctor yet. Although I did make an appointment for the 23rd on this month. Any ideas what could be going on?