I've had the same thing happen to me except I wasn't using the bluetooth either time. Just Navigation and received a call. After a couple minutes power reset. Battery was at 75% or better when this happened.
This happens to me, but I have found that if I have the google navigation screen on top after making or receiving a call then it doesn't happen any more. Seems to be a issue with google maps working in the background during a call. I am a truckdriver so I use it this way alot.