pain in my back

this am, i went out for a quick run and just as i was finishing my workout, i think i pulled a muscle somewhere between my waist and shoulders in my back. i wasn't pushing myself in any way- i was running at a medium pace. i recall that it happened as i was running and attempted to take a deep breath. it is a little painful, especially when i try to breath deeply or when i stand up from my desk and try to stretch. has this ever happened to anyone? what to do?