How can i stop back pain from playing the violin?

Lumbar support: Your pain is likely from the posture you maintain while playing or from sitting for long periods of time. Be sure to stand up every 30 min and stretch your hamstrings. You may also want to obtain a lumbar support to place in your chair while playing. This will reduce the stress placed on your lower back.

Neck stretches...: You should use neck and thorax/ribcage stretches. Usually the position of holding the violin can put strain on the neck and upper thoracic muscles and other soft tissues. Stretching these out will make a real difference in the overall comfort of playing. There are some very good visuals out there on the web which will demonstrate good stretching techniques.