A 2cm difference will produce a notable difference. At 5' 10" on a medium, I'm inclined to think you might want to go even longer (120cm). Also be sure that your saddle is level or even slightly nose up to prevent you from sliding forward and causing additional weight on your hands. One more thing to check is your handlebar rotation. If your hoods are angled down, you'll be trying to grip tightly to avoid sliding forward and this will get tiresome quickly. Handlebars can slip in the stem clamp and wind up at all sorts of wonky angles, and some people set them up that way for reasons unbeknownst to me. With modern shifters, most aim for a flat transition from the tops of the handlebars to the grips on the shifters. You may need to slide the shifters on the bars to achieve this, and depending on how much you need to move them it could require removing the bar tape.
Start with the stem, though, as you likely will benefit from something longer than 90cm. I'm 5' 8.5" and on most 'medium' size frames would use a 100mm stem.