I have been riding my road bike for a year now. I decided to flip the stem so I can be in a "more agressive" position. It's not a huge difference but I really like to be able to get down lower. However, not unexpectedly, I have found my shoulder blades/upper back is tight and sore after the first couple of rides. Is it just a matter of getting used to the new position? Do I need to get my fit rechecked now that I'm in a bit different position? Do I need to go back to the way it was?

Any thoughts?