It sounds like you haven't been training your back, it is the most important part for any athelete to train.

You probably have a weak back, at age 13 I would suggest doing light weight high repitition exercises such as:
Back extensions on the floor.

Performing light weight, high reps will increase your slow twitch muscle fibres (the ones that are harder to pull and damage).

this inturn will mean that you will be more unlikely to suffer from back pain.

Also exercises such as crunches and situps help protect the back (I would stick with crunches until you are sure your back can take situps).

(These exercises will sort your posture out anyway - although that is not the aim here).