Makes my phone shut off!

Three separate occasions now- while using the app on a walk, my iPhone 6 has powered itself down and then wont turn back on. Screen only shows the "plug in" prompt when I try to turn it on like its saying theres no battery power! But, my battery is always at least at 50% before I leave my house! I get home, plug my phone into the charger, and only then will my phone turn on- but when it does it shows the battery wasnt drained at all- still at at least 50%!! VERY FRUSTRATING!!! I wont be using the app for my walks anymore.

thatgirlstef about Map My Fitness by Outside