Why do my shoulders and upper back hurt?

Well I haven't done anything to hurt them. Whenever I move my arms back it starts hurting my shoulders and my upper Back and when I lean forward my back tightens or tenses idk. Could it have been caused from sleeping the wrong way? And now it hurts even more to walk. I slept sideways so idk if that's the problem? Becase after sleeping sideways I woke up and now I'm in even more pain. The pains mostly on my upper back.