I play baseball an my lower back of my throwing shoulder hurts when i throw an extend my shoulder backwords in a throwing wised position isit serious?

Likely a sprain: More than likely this is muscle strain, and can be treated supportively with Ibuprofen and rest. If you do not improve with this treatment you should see your doctor. If you develop tingling, numbness/loss of sensation, loss of strength or shooting pain down your arms or back, you should seek immediate medical attention.

Probably not: It is probably not serious but you should always see your doctor to make sure. In regards to pitching it is very likely that your throwing mechanics are off kilter and needs some fine tuning. Sports specific physical therapy is a must if you are an avid player. I deal with baseball players daily and I do refer all my pitchers to a overhead throwing therapy program