Cycle Travel overheating my phone

I'm a new user, trying our Cycle Travel on local routes before using it in earnest when we go to France in the summer.  Today when using it on my phone, after only half an hour I got a message "cycle travel closed as it is overheating your phone".  I was not able to turn it back on for at least 20 minutes.  Any ideas?  It was a fairly warm spring day (22nd March in southern England) but nowhere near as warm as it will be in summer.