It's a food bug. I'm not totally sure how to fix it, but it happened to me the other day as well. Tri-Foods and changing zones (especially moving to and from Cyrodiil) seemed to trigger it for me. Logging off, eating a different food, then eating the tri food I wanted fixed it once for me, but it hasn't since. It feels pretty random. Also, it affected Health and Magicka as well, not just stam. Really wish Zenimax would get their act together when it comes to coding.