Back/shoulder pain: ideas for stretches or other fitness-y activities? →[More:]

I'm commuting five hours a day right now, sitting down on buses and trains. It's awful, but I get a lot of reading done, so mentally, I can be rather Zen about it, my students are a blast, and my salary actually reflects the huge hassle this is, which makes it worth it. This too shall pass and all that. It's an adventure.

But: ouch! My back is really painful most of the time now, especially the right side of my lower back and right leg, which is opposite where the strap of my heavy bag hangs on my left shoulder.

I've tried switching shoulders, but they seem to be quite unevenly muscled (?) and my right shoulder can't handle the weight as well. I caught myself limping the other day!

I've got to carry about 5 kilos/11 pounds of materials to and from the lesson site, and there's about 2 km/1.2 mi of walking I do with the bag between home, buses, trains, and the site. Poland is also ALL stairs.

What can I do to relieve this, in addition to trying to switch shoulders? Hot showers help, chamomile tea helps too, but I'm looking for, say, a yoga pose or a great stretch I can do to either limber up for the lugging of materials or relax the stressed area through the day. I could take a painkiller but I'd rather see if there's a non-medicinal way to deal with this first.

A couple addenda:, this all ends in a few weeks when I move to a new city, so joining a gym is out, unfortunately. I also don't own any sneakers/trainers at the moment, so while I do walk all over the place on the weekends (without a bag), I can't really go for long runs.

Dzięki!