Playing the violin puts a lot of stress on the neck and shoulders. The people who are most severely affected are grown men with large muscles, but everyone who plays the violin is affected to some degree. The problem is most acute when you're first learning to play. You should stop practicing when you begin to feel muscle tightness creeping in, even if you've only practiced for 5 minutes. Then stretch your muscles to relax them and resume playing only when your muscles have relaxed.
The neck muscles work hard when you play the violin. Here are some good stretches for the neck.
If you have more serious, complex problems with your neck, as I do, the following video is for you.
Other muscles that work hard when you play the violin are the shoulder muscles (deltoids) and pectorals (pecs), which are on your chest near your armpit. These muscles are usually the first to cramp up when you play, especially if you're a beginner. If you're an adult male beginner, you'll really feel cramping here. Here are some good stretching exercises for these muscles.
When you try to watch the following two clips, you may get an error message telling you that the video is restricted from playing on certain sites, and they don't play here. Just click on the center of the picture, and you will be redirected to the video on Youtube, and the video will play.
For more on stretches for violinists, please read one of my blogs on violinist.com, http://www.violinist.com/blog/paulinefiddle/20099/10489/
Here's to your health.