I was in fuji ryu ju jitsu two years ago,I left for two reasons 1. my back started to have pains which still some time come back... 2. My will to learn martial arts must not have been strong since I stopped going.(I'm planning to go back) But anyways back then i didn't do anything to overstress my back(I'm 13 right now) I don't know what cuased my back pains...then last year i started to do weight training.... i wasn't using bars just dumbbells... but my sister says i overdoing them so i slowed down. Well anyways in school I do any extra curricular thing that has to do with weights or pylometrics. But anyways I was wondering if anyone knew what could of cuased my back pains.
PS: My dad and my sister say it's bad posture. And as of today i still get the pains back once in a awhile.

It may be bad posture but I doubt it. I have horrible posture and no back pains. I would say you stretched something but it wouldn't have lasted this long. Did you get hit in the back during your training? Or had your back been injured before starting?

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:
Deadlifts:
Goodmornings:
Hyper-extension:
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).