Have the same problem.. Are you using skyre? I got the bug before and had to reload an earlier save, today I got the bug again but this time I noticed that it happened when I got hit by a bandit using a crossbow bolt that slows me down, I got hit again and sped up then I wiped out the whole bandit camp and noticed that my character is now moving super fast.. I'm playing a vampire with better vampires mod btw.