Painful shoulder from treadmill exercise is putting me off going back

I was on the treadmill about two weeks ago, doing a burst of jogging/running during my walk and I started feeling a lot of pain coming out of my right shoulder. I ended up having to cut the workout short because I just didn't feel right. It's been quite sore on and off since, and the current theory is that I pulled a muscle. I have only been back on the treadmill once since, and I definitely felt it again then, so now I've grown very wary of going back. I have a lot of problems with my arms already, with RSI in my forearm muscles making me extremely aware of any sensations going on in them, so I'd rather like to not add shoulder problems to them. I'm not sure the muscle has healed, I can't yet leave a shoulder bag on there without it starting to hurt a few minutes later.

My question is, a) how the hell did I pull a shoulder muscle by jogging/running?! and b) is it likely that I'll get sore again if I go back on the treadmill? If so, how soon would be safe to return? I only started working out for the first time in February and I'm worried that if I don't keep to a semi-regular schedule, I'll just drop it as I've dropped so many other things.

I really didn't think pulled muscles took this long to heal!

Some shoulder pain when running is not that uncommon, actually.  I get it from time to time in my right shoulder. 

But the cause is straight-forward:  I am too tense when I run.  When I start to feel that pain, I lower my arm and shake my hands to ease the tension.  You want to run loose with your hands in a very loose fist - basically just touching your fingertips to your palm, not tight.  You want your shoulders to swing freely rather than feeling you're muscling them back and forth.

I don't know if this is your problem, but pulling a muscle in your shoulder while running would be pretty surprising if it wasn't because of excess tightness.  Try loosening up good before running, and when the pain starts to creep in, relax, relax, relax.

That sounds about right, actually. I tried to relax but my tendency when running is to use my arms as part of the way to pull myself forwards, if you get me? Putting everything into every part of moving forward and keeping going.

So, now that I know NOT to do that again, how long til it's safe to go back in the water? :)

Well, I'd try it right away.  As I said, get warmed up slowly and get a lot of movement in that shoulder before running, and then ramp up your speed and let those shoulders "drop" and your arms swing fairly freely.  Keep your elbows bent at around 90 degrees, but don't force your arm swings.

See what works.  For me, I just run through the pain - when I do that, it goes away.  If I stop when it hurts, then I'm in trouble...

Seriously,,,,take 1 "Advil"  BEFORE  any exercise! GOOD LUCK!

DON'T QUIT ,,,NOW  !!:-)