I've been playing guitar since last year and know a few arpeggio based songs but I unfortunately find playing these songs becoming more of a chore because of pain in my right arm when I rest it on my guitar. When I start the song my arm feels fine then an ache starts in my forearm then my arm begins to hurt to the extent that I have to stop playing and reposition my arm. I am pretty sure I am holding my guitar correctly so some advice would be greatly appreciated!
I actually just had that... my advice: see a doctor if it keeps up. Record the times it hurts and see if there's a pattern because that will help My right arm started hurting around Christmas Eve and I had to keep it bandaged, and it gradually became worse because while it didn't hurt all the time, when it did the pain was worse each time and it moved up my arm. My doctor said I have carpal tunnel syndrome and a lot of guitarists get it, it can eventually lead to major issues if it's not treated, though :/ I have to wear a brace right now, but I can take it off to play... but if you want to be able to play for many years to come I would go see a doctor about it. I will be praying for you! I know how hard it is If there's anything else you need don't hesitate to ask!
No problem! I would suggest using a bandage while you're playing from here on out... you're gonna think I'm crazy, but try finding a wrist support bandage like quarterbacks use It's just a simple wrap-around bandage which only wraps around your wrist and that you can slip on and tighten. It helps a lot, just keeps your wrist straight which will (hopefully) reduce the pain... and take Ibuprofen afterwards even if it doesn't hurt!
It is possible, and my first guess, that you are developing a repetitive strain injury from playing. This can happen even with proper positioning. Some links regarding the subject;


If you play with any sort of regularity, or for any extended period of time, it is increasingly important to warm up and stretch the muscles and ligaments in BOTH your hands and arms. As you log the years of playing, these repeated motions build up, and can begin to cause pain and discomfort.

There's a million different routines and ways you can warm up, so either develop one for yourself, or YouTube an established one; it's no different than stretching before phys ed class in grade school.

I hope this helps, and regardless of the cause you're able to find a solution and get back to doing what you wanna do.