[Biker1]

Just an idea. You may want to discharge the battery fully, until the device turns off and then remove the battery, reinstall and then let the battery charge fully again, soft reset and see if that fixes the problem.
I read somewhere that when the device is new and used but the battery is not charged fully, the device does not properly read the power left, or something to that effect. But you have been using your device for awhile so that issue may not apply.
Something occurred in the way the device is reading the amount of power left, whether it is a new application which was recently added or maybe the device just needs a soft reset.
I don't have a definite solution but maybe someone else does.