Sore left shoulder after 20 miles
I have an issue when riding my Madone 4.7 where my left shoulder becomes very sore after 20-30 miles. I try to move it around every so often on the bike, change positions, etc. to help it along. I did two century rides last year on this bike and it bothered me then, although taking rest stops definitely helped. It feels fine after getting off the bike, but while riding it gets to the point where to rotate my shoulder backwards becomes very painful... but this is my left side only. I have checked the alignment of the bars, and had my LBS double check all of that, and there is nothing wrong with my bike. I have also completed several longer rides on my other bike, a Trek 7.2fx with flat bars.
Initially I thought it was something I would get used to, but I've ridden this bike about 1500 miles now... I have fiddled with saddle position, tilted the bars up slightly, etc. The bike feels really good otherwise.
So my question is, outside of getting a professional fit, would shortening the stem help? I currently have a 100mm stem with 7 degree rise. Would possibly going to 90mm help?
I figured I would try this before going all out on the professional fit... Stems are much cheaper! Any advice would be appreciated!