15 Year old Male experiencing pains on left side of chest

Hi, I got to the point were I was so worried that I had to look somewhere to find out what was wrong with me.

Im 15 years old and is extremely active.  I play sports such as football and lacrosse and i'm a very fast kid.  I started experiencing pains on usually left sometimes right side of chest, but whats weird is that i don't experience these pains when im playing sports or am active.  I only experience these pains when im sitting around maybe playing video games or going to bed.  The pains aren't unbearable but they are noticible and I wonder what could be wrong.  My parents learned today that I felt these pains and they scheduled me for an appointment soon.  Im pretty nervous about that im going to hear which is why I was hesitant to tell them for so long.  I've been experiencing these pains for a few weeks now.  

The pains are usually a little sharp then go away then even leave for a while but then come back.  Some are sharper than others and if I breath in a giant breath and hold it when I have the pain I can feel some more pain.  Im pretty scared about this because I dont want to stop playing the sports that I love.  Im not over wait what so ever I work out on a daily basis for football season coming up and im on a very good diet.  I don't neglect vegetables and fruits but i do enjoy junk food once in a while im about 135 pounds.

Hopefully someone posts soon about my problem, thanks.