Theres a sharp pain in my chest?

For the past few weeks if I'm sitting down for a while I feel shap pains in the middle of my chest. They mostly come from laughing or breathing. About once a day. Mainly on walks I suddently cant really breath cause it hurts so bad. My friends think I'm faking it sometimes but today I was on a walk with... show more For the past few weeks if I'm sitting down for a while I feel shap pains in the middle of my chest. They mostly come from laughing or breathing. About once a day. Mainly on walks I suddently cant really breath cause it hurts so bad. My friends think I'm faking it sometimes but today I was on a walk with them and suddenly when we where laughing it hurt really bad and I couldn't really breath. When this happens it lasts from 5-15 minutes but I still feel slight pain for hours after. I'm a 13 yr old girl. Very active and dont have asthma. Please help if you have any ideas what it is. I really want it to stop.
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