What do i do about a pulled muscle?

at least that's what i think it is. i was heading out of the store yesterday and when i lifted the bag of dog food actually it was 2 20lbs. bags of dog food onto my left shoulder i felt a tearing pain in my right side the pain started about 4 or 5 hours after that i took 2 aspirin that night and 2 this morning. but the 2 i took isn't doing much for the pain it hurts every time i move what can i do to lessen the pain?