Hand/arm numbness can be caused by neck/shoulder problems, aggravated by bike position, especially if you have too much weight forward.

Also, consider riding without the messenger bag. Just having the bag there might be adding to shoulder/neck tension, and contributing to the numbness, especially if you carry it with the strap over your right shoulder. Plus, alot of computer/mouse time can add to problems with chronic neck/upper extremity tension, IME.

Try a typical numbness-causing ride without it and see if it make a difference.