I am 22 yrs old and have chest pain OFTEN?

I am a single mommy, college student and worker. Everyone says stress but I am good about controlling it. I am a little overweight and I am in the process of quitting smoking...(ie i've cut down A LOT) I uste to always get pains in my left side (rib cage going around to sternum) exactly 2 wks b4 my period....now i get it whenever, w/o warning. I am healthy and faily active....My doc ALWAYS said plerasey but I'm begning to worry. It hurts and it scares me. It'll go away and I'll be fine then a week later it'll come back and last a while...then go away then come back the next day....it really varies....WHAT is it? (ps I'm going to the doc again to poss. do a stress test??)