Are you under a lot of stress? Do you use your voice a lot?
Sometimes things like that can affect the throat muscles.
Stress sometimes can cause the throat to tighten/constrict, even esophageal (sp?) spasm(s).
Try to see an ENT, just to rule out any infections,allergies,polyps,etc.
Lots of luck with this, let us know how you make out if you wish to share with us.