About a month ago, i broke my collar bone, meaning that i couldn't use my strumming hand until about a week ago.
When i play now though, my arm gets kinda dead because i haven't used it for a while, meaning that after just a little serious tremolopicking or alternate picking, my upper arm is done. Before the injury i wasn't what you would call a strong man, meaning that i even then would often get a tired arm from playing very hard.
So i wondered, what kind of drills would be good to get my arm in shape? Standard weightlifting, or should i be more alternate?
Weightlifting does nothing for your guitar playing. Your trying to find loop holes to get better faster when really your wasting your time and the simple answer is... practice.

If your arm is hurting it's because you have bad technique, picking from your wrist (or thumb + finger motion) without tension.
I'd say work slowly before you do yourself an injury you can't come back from. Forget tremolo picking at speeds 120 and beyond with anything like 16ths... Forget completely about 32nds at 60 too. Iow... take your time dude...

You're talking about not having used it at all in 3 weeks... you aren't even properly healed yet. Play some rhythms.... do something constructive. Make some music... as time goes on, your stamina will return (after you have built it up from scratch of course)...

Otherwise, ask sean... he had kind of a worse injury (tongue in cheek though... I do not envy a broken collar bone at all!!!)... but its still an injury. He might be able to give you some tips as to how to rehabilitate yourself.
It isn't even always from playing fast, it's often just from playing for some period. And now im worried that i'm picking all wrong When you say tension, what exactly do you mean?
"Tension" in picking is a wrong physical mechanism that make the muscles of your hand/wrist/arm to tighten, causing a physical limitation to the lenght of your playing and your precision and in the long term could cause tendinitis. Which is ****in' bad, trust me.
Building your muscles up is not a bad idea. You should mostly use the muscles in your forearm for your picking technique and thus you can do a bit of exercise for them. I have a thing called Powerball and it works wonders for the muscles in your arms.

Your arms should be relaxed all the time regardless of what tempo you play at. Try picking slowly and build up your speed. If you feel tense as your raise bpm, practise at a lower tempo. Playing should never hurt. If you experience pain, stop immediately.

In my opinion practising downpicking is a good exercise for you muscles and does good things to your guitar playing. Start out ridiculously slow and examine. Try not to make any movement from your elbow. Wrist all the way.