My muscle between my neck and shoulder hurts really bad when doing excercise, it's been happening for about 4 months?

Try ice, stretch: Sounds like a muscular problem with tightness i would try ice and some stretching exercise be more aware of your posture you might benefit from a period of physical therapy.

SHOULDER / NECK: This problem may be from several causes, ranging from degenerative arthritis, cervical disc derangement, simple muscle pain, or improper mechanics during exercise. You need a baseline workup with your family physician, starting with a thorough history and physical examination and probable xrays.