Young man, while caddying may or may not have any cause as to why you're experiencing this unexplained pain and discomfort, I will offer this very wise and urgent advise to you.
No one on this board is a Sports orthopedic, and even if they were they would not diagnose what your problem is without seeing you in person. You will receive many "suggestions" as to what it may be but none of what you hear might not even be close.
Have your parents call as soon as they can to make an appt with a good Sports Orthopedic in your town, or town nearby. They are trained in this and can diagnose exactly what is wrong. I'd also suggest stop throwing and even caddying until you have answers. To continue on could only make things worse.
Good luck. Let us know what you find out.
YGD