I used a 20lb weight and I didn't feel pain
but I had some fatigue in areas and I could feel my shoulders tensing, my wrist/forearm fatiguing and bicep. Holding weight while in the first position I also felt my shoulder impinge by deltoid (maybe my unique problem).There is a tendon that goes from the shoulder to the elbow (hand actually) this is quite easily to get tight. If you put your hand in your armpit but near the bicep underside you will feel these (inside of shoulder) Try get in position or just straighten your arm to your side and a little front feel the inside of the shoulder and if you feel "string-like" tendons in the inside of the arm let me know. There are stretches that can help stretch those tendons out. They are weak so if you work them while playing guitar, you will not last long. Sound familiar?
Honestly, it's almost impossible for me to tell over email, but I can tell you the things I've been through. I'm a teacher with tons of students and deal with these things all the time... I wish I can see you! Can you video tape it or send pics? Show us where? Scott Tennant actually was the man that helped fix my position years ago that would have prevented my problems (note: when these elite players are in town, go see them or perform for them!). Maybe that is your cause...
Do you feel like your over extending to reach first position? Does it feel better to put the guitar on the your right leg? You said it's fine around the 5th position? All this leads to having the guitar most likely in the wrong position according to your body's mechanics... it's each to their own mile... I have LONG monkey arms and have a tough time with position.
Can you take a pic? Where's the sound hole of the guitar is you're holding it in classical position? Is it near the center of your body, slightly to the left (if you're a righty), or away from the body?
Is the angle of the guitar close to being 90 degrees? Is the left foot
pointed straight? Where's the left food? Is it in line with the inside of your left shoulder?
How tall are you? What is your age? Any history of trauma / injury to the left side of your body?
Stretching helps but you will NOT get immediate results... well maybe not. Stretch your chest, shoulder (upper capsule, supra spinitus, lats), bicep, and shoulder to hand tendons. Do that by putting your hand flat against the wall with your side to the wall. Turn your head to the opposite direction and it should stretch. If you feel any stretch... that muscle group could be the problem. Upper limb/shoulder/neck problems are tough to diagnose and often times one pain
in an area (say shoulder) is caused by something else (humorus pull).
It's worth a shot and I gave you a lot. I hope something in there helps. Worse case, just hold it on your right leg flamenco style and see if that works. Elliot Fisk does it all the time (and Paco De Lucia!).
Best of luck, I hate to see you stop guitar because of this.