I've been having a persistent pain on my left side of my back a little below my shoulder blade.it has been a month.what could be some causes for this?

Muscle Spasms: Persistent pain on back near shoulder blade on one side is usually muscle. You don't really need a reason to have back pain. Back and neck muscles are small and difficult to stretch. Also problems can begin so slowly they go unnoticed for months - even years. Often when first noticed back muscle pain has been present at low level for a year or so. Likely other muscles/areas involve too