For some reason when playing standing,

If i have the guitar higher up, my back and neck gets sore and my picking arm gets sore from being higher up

lower and everything is fine, except for playing things at around the 1-4th fret my thumb sorta goes toward the headstock and becomes... completely straight and not relaxed,

Is this normal?, it's not really giving me any pain but it just feels.. odd sometimes does it when playing the middle of the neck as well.

is this bad? or should be fine?
how long are you playing for until you get sore?
where is the guitar placed on your torso?
your thumb stiffs up and points into the neck around the 1-4th frets?

"Hey kid. You wanna cigarette?"

"No thanks! I/m already hooked on Fonicks!"

like 5mins my back and neck will get sore and my arm as well just from how it's sitting and where my arm is while trying to pick,

when it's lower i'm relaxed and it just feels more natural and i'm looser etc

and by my thumb i mean instead of just being relaxed and normal it goes stiff and bends backwards

instead of having a slight foward curve when it's relaxed it's like bent backwards and from the joint in the middle

If that makes any sense.