I’ve recently started playing with a pick and I’ve found that after playing for a few minutes I begin to experience an “ache” in my upper arm, do I just need to get used to playing with a pick or am I doing something horribly wrong?
I think its probably the angle you holding your arm at. Is it higher then normal. Maybe you could mess about with different strap hights And see if that helps.
fatgoogle is probably right but when i had the same problem i just laid off the bass and played the guitar for a week or so and it hasnt bothered me since.
Are you doing all down strokes and moving from your arm and shoulder? If so, stop doing that and use a more efficient picking technique. If not check your arm angle as suggested above.
Make sure you don't have ANY tension in your picking arm...this can lead to tendinitis and other repetitive-strain type problems, in addition to the aching you mentioned. If you can get your arm and hand as relaxed as possible, the aching feeling should go away.