Sounds like you may have a pinched nerve (inflamed... improper stress) which can be somewhat common. If noticed from the neck, traps and upper back, than chances are that is what it is. Turning the neck or lifting the arm too high may also cause discomfort. Usually confused with a actual muscle strain, though both can go hand in hand. Trying to work around it if you do not want to skip a few workouts. Will usually go away after 3 or 4 days. In any event, warm up completely before any workout. Hopefully a pinched nerve is all it is and not a muscle strain (rupture/tear).
In any case, apply a heating element (lotion, gel, taped heating pad, etc) throughout the day. If you can afford a professional massage than that can be an excellent cure. If not gone away with-in a week than see a doctor. Who, in most cases, will subscribe a anti inflammation drug(s). And advise total rest for a week or two. Good Luck.