Why do i keep pulling muscles?

You are not in condition....

you should practice stretching, and

avoid doing things which are

extraneous to usual movement.

You are likely growing faster than your muscles,

and will experience all types of

aches and pains up into your older teen years....

it is quite common

You should try stretching more before doing sporting activities, Your muscles are tight from not warming up this could be why you are pulling them.

Make sure you do a warm-up if working out, and always stretch good after

Too much exercise, have a hot bath, relax a bit more, don't tense your body

Stretch more