Pain while running, information/advise?

I am a competition runner and rack up a lot of mileage during the week. Sometimes during my run I get a sharp pain in my upper center chest. When this happens I try to slow down or even start walking and it goes away in a short amount of time. This pain doesn't radiate to other areas of my body or throb. I even get this pain while relaxing, not often at all though, probably once a month or two. Should I be worried about this? I talked to my doctor a while ago about it and he didn't say much about it. Any information or advise would be great on how to avoid this or what the cause may be. Thank you guys!