Once in awhile you might wake up hurting really badly and you don’t know why! It’s often because you did something that stressed your muscles, tendons or ligaments even days before, although you didn’t experience pain at the time. Ankles are often the victim.
When that happens, take it easy. Lie back, with the offending limb elevated. Add a cold pack and watch some screen or read a book. Then bind the poor ankle in a good old elastic bandage when you need to be mobile. R.I.C.E. Rest, Ice, Compression, Elevation. Repeat throughout the first day and until any swelling subsides.
Don’t do more than you can do because the odds of re-injury are very high. If needed, use crutches for a few days until you’re stable and able. If it’s a serious sprain and more than just a strain, you should see your doctor. For more details on self-diagnosing: https://www.mayoclinic.org/diseases-conditions/sprains-and-strains/basics/definition/con-20020958
Therapy cats or dogs can help enormously.