Willy,
Certainly sounds as if you have a rotator cuff tendinopathy (fancy word for tendon pain / problem). If the problem has been there for more than 2-3 weeks it is unlikely that anti-inflammatories will help, but you can try them. If no relief after 4-5 days then stop taking them,cause they ain't helping.
The massage that i described will give you some relief. In the meantime find yourself a golf ball and place it in the middle of your shoulder blade (flat triangle shaped bone known as the scapula). Press the ball between your scap and the wall and roll around until you find a point that makes your eyes water (at least). Hold pressure over this point until the pain reduces by 50%, then move onto another. there are usually 2 main ones closer to the middle of your back and one in the middle of your scapula. If you have access to heat then put some on otherwise hot shower will help.
Tiger balm and pain reliving creams will also help - as will some asian arthralgia patches. Chilli patch or musk patch - they look like a big band-aid but are impregnated with liniment and herbs.
There are exercises to avoid - those that hurt (common sense i know). Stretch your neck and upper back. You can try a rotator cuff stretch as below hold for 1 min repeat 3 times 3 /day.
basic strengthening is as follows
Now it unlikely you will have a theraband - so modify this and lay on your good side and perform the movement holding a 1 kg weight and slowly increase the weight as your strength increases 3 sets of 10-12 reps twice per day.
This should get you by until you make it to a western country, where you can get to a physio. For a little more info check out the following link - this will have a few more exercises as well.
au.yahoo.com/?p=usBest of luck. Enjoy vietnam - try a balau if your game

Darryl