having chest pains lately need some advice

I've been smoking weed for almost 3 months now. twice a week. Sometimes ill be perfectly fine but ill breath in heavy and it feels like in the center of my chest a like stretchy muscle kind of pain and then it'll go away. Its like a 3 on a scale from 1-10. I am in really good shape with a low cholestorol for a 15 year old and I don't want something bad to happen so please some advice. I want to know exactly whats happening to me, thanks!