Occasional pains in the left side of my chest

I've been having this for 4 months now. It started when I tried out smoking a little 5 months ago, (2-3 cigarettes a day at most) and then suddenly after a month, when inhaling the smoke, I got a huge pain in the left side of chest that feels like someone stabbed me in it. It comes and goes, especially when I breathe hard.

1-I never smoked before, only that month and just so little. 2-The pain is still ongoing from time to time and for some reason came back in the last week.

So what is this? How dangerous is it? And how can I tell if it's in the heart or lungs? When should I really worry about it?