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Absolutely the best advice on the subject.
If you live in a big city, you may even be able to see a doctor who specialises with musicians. (There is a musician's clinic here in Toronto). You could also try an osteopath, physiotherapist, or maybe an Alexander technician and bring your euphonium with you. They will be able to diagnose your problem by observing your playing posture.
The generic advice you will most likely receive is:
-Practice sitting down at least 50% of the time.
-When practising standing up, put down your horn and rest for a minute every five or ten minutes.
-stretch the affected areas. (the doctor will be able to show you the right stretches).

Your situation is extremely common: you have pain due to a posture that creates an imbalance of tension in your body. I think just about every serious musician goes through this situation at least once and probably several times during their career. However, every person's body is unique and the exact recipe for relief will depend on your own personal circumstances. Which is why you MUST seek professional advice.

Good luck!

Check out someone who does the Alexander Technique.

If your arm is raised above your shoulder that will irratate your shoulder. Stretching and ice are also very helpful; a massage or a chiropractic adjustment may also help.

Ultyimately, if you are frequently in pain I think the most effective adjustment you could make would be to have your teacher and an Alexander Technique therapist evaluate your playing position. When I was in school I met a saxophonist whose back injuries nearly ended his chances of playing; Alexander technique brought him back. . .

Good luck.

The Alexander technique is a series of bodily movements carried out by a secret lodge.

You are initiated to the lodge, when you are the original buyer of an Alexander wind instrument.

Unfortunately I bought my Alexander horn second hand.

Klaus

Who stands when they practice??? It's like marching band or something... Have a seat!