What Causes Shoulder Pain?

Last Wednesday, I strained my shoulder reaching for something that was too high for me, it had been painful up until today, when a friend asked me to climb a brick wall that was half a foot taller than me. I ended up hurting it once more. It hurts when i move it in any way shape or form, when i lay on it, and also when i pick up something that is more than 4 pounds. What exactly did i do?