Shoulder pain caused by messenger bag? I have a professional messenger bag (chrome mini metro messenger) I have been carrying for about 2 months. The bag weighs around 3 lbs and I generally carry about 4 lbs of stuff in it. Biketools, phone charger, gloves etc. Its less than 8 lbs and I still get shoulder pain. 

I am 190lbs and 5'10

Am wondering if there is something I am doing wrong - I see a lot of messengers in the city with huge bags and always wonder if they have shoulder pain from carrying bags on one side for years.

Below the actual bag although i have it higher up without using the side strap.
Take a look at your riding position.  When I was much younger (about 10-12 years ago) I used a messenger pack all the time while in an aggressive riding position (e.g., track bike / road bike) . In that position I found your lower back supports most the weight of the bag.  In a more upright position it pulls on your shoulder more.  As a test, try walking around for a while with one... it sucks.

If you prefer a more upright riding position (no judgement!) then a messenger bag may simply not be the right fit -- sorry pun intended, my bad.