Tomo Fujita Guitar Wisdom

I've been learning guitar for a month now. I was given a guitar a few months ago and realized 3 weeks into my learning that it is in fact a classical guitar. This doesn't bother me since it was free. Since, I've learned there was a difference, I found out that I should be holding it in the classical position which I learned yesterday.

Today, however, I've noticed that just a few minutes into practice my upper arm (in the shoulder area) is quite fatigued. My fingers and wrist are fine. (Edit: Feeling some discomfort in my wrist now as well) I'd like to know if anyone has any pointers or tips for me to improve my posture maybe and relieve this fatigue.

Beforehand, I was playing this guitar like a standard acoustic and could practice for an hour or two with no issues. I did have to make up for the fact that this guitar is small though so I'd prop my legs in a way that would support holding it like a standard acoustic lol.

Maybe, I should continue holding the guitar like I did previously? Thanks for your time fellow students!

RELAX
You are experiencing pain from tension
Try focusing on relaxing your arm more while in the playing position
Classical guitars have wider necks and can lead to fatigue faster
Especially as a beginners we tend to be more tense and tight while trying to practice
Try to only put the necessary amount of pressure with your fretting hand to sound the note or chord you are playing applying too much pressure is a common source of pain and tension

Here is Tomo's comment:

"Thanks so much for subscribing to my Guitar Wisdom. Posture is so important! Straighten your back, and don’t sit on a couch!
You should not continue to practice, if you have ANY pain. Use a strap! Set it so that the guitar is a little higher on your body."