If your not starving or dehydrated & not bleeding then try seeing if your food and water bars have any red downwards arrows whilst at walking speed. I've had that happen a few times , sometime my character bugs out and gets locked into a sprint response even though I'm not sprinting. Logging and joining a different server has always fixed this for me. If that's no help then maybe you've taken a considerable fall, falling can cause a broken leg which can only be fixed by combining sticks (harvested with knife,machete, etc) harvested from a bush with rags to create a splint. Apply said splint. Ive also had a similar issue but didn't know how to craft a splint at the time. Hope this helps
