Are you sure your legs aren't being broken? and are you picking up to many objects?
1. Being attacked by large enemies like bears, and being hit by blunt weapons (Club, Mace) can result in broken arms and legs. A broken leg can slow you down greatly, you'll know the leg is broken by a red icon near your health bar or by pausing and checking the left side.
2. You might be carrying to much stuff. Your backpack has a weight limit before it slows you down.