What Causes Stiffness On Left Side Of Neck?

I HAVE A STIFFNESS ON MY LEFT SIDE OF MY NECK AND I HOPE ITS JUST A MUSCLE BUT IT FEELS DIFFERENT, AND IT FELT AS IF IT TRAVELED TO THE BACK OF MY NECK... MY QUESTION IS CAN I DEVELOP A BLOOD CLOT WITH OUT GETTING HIT THERE? OR GET ANY TYPE OF BLOOD RELATED THAT WOULD HARM ME?