Suggest Treatment For Sore Muscles On Left Side Of Neck

I have sore muscles on the left side of my neck extending down towards my left shoulder; almost like a soreness from when you sleep the wrong way, but this has now lasted almost a week. Aspirin helps the pain go down but it constantly comes back; I haven't had any extremely strenuous activity (in fact when it started i had been sitting for a while) where I had pulled anything. Any suggestions?