Pain/Pressure/Tightening in my Sternum when I laugh?

Whenever I laugh, even if it isnt particularly hard, there's a pain in my sternum that feels like my chest is tightening up. I haven't done any strenuous activity lately, and I'm 13. Its only when i laugh, and it isnt tender to the touch. Please help me, nothing i have found online fits! What's going on?