Tight chest/breathing?

Hey im 17 years old, i weight 180 pounds im 6,0ft and i usualy dont be active, maybe a walk to the store or somthing here and there, but every 2 weeeks i mow grass its about the size of a soccer field and it takes 6 hours, now today i feel somewhat tight on my chest and my breathing it isnt horrible or painful or anything, but im a worryer and im just curious as to what could of happend or whats wrong also, it feels a lil different breathing after drinking water, please voice your opinion and help me thankz :)