What Causes Chest Pain And Shortness Of Breath While Working Out?

Hello, I have been consistently working out for two years now 5 days a week. All five I do cardio and three days a week I life weights. So far I have been able to run without stopping and without being exhausted or overly tired for three miles. Sometimes however I develop a kind of bad cheat pain on my left side about 1.5 miles in. I have no other symptoms of tiredness and sometimes if I breath extremely deeply the rest of the time it will go away. Is this cause for serious concern? I also do very hard elliptical workouts which always by the end I am more tired than when running but the chest symptom never occurs and I do the elliptical nearly every day.