I'm not fat or super fit but i get out of breath easily, why?

I'm skinny but not an exersize maniac, so i don't really do any sports, i have a very healthy diet and i can run easily and when i am running i don't get out of breath, even if i sprint for about a minute, but if i run for even 20 seconds then stop, i get really out of breath. I don't know whether it's because of the cold i have at the moment, or hayfever or whatever. Does anyone know what it could be?