Likely related to MCPE-9807
Can you confirm that you weren't recently sprinting when you notice this?
I can confirm this bug in the current Windows Store build. (0.12.0) Though the description may be a little inaccurate.
When you begin sprinting, you will continue sprinting for exactly 30 seconds, or until you walk into a wall. Particle effects included.
Example video: https://youtu.be/WZSo5Bpv9tc (Note the particles at my feet.)
If this has been fixed in a later build, please change the resolution to "Fixed". Otherwise, reopen it.
Duplicate of MCPE-9807