That's practically guaranteed to be damage to the display element. If you have hard objects in your pack that pressed "heavily" into the screen, or heavy but blunt impacts you could end up damaging the underlying display without cracking the glass. A heat source might do the same thing. If you damaged the display element, you've probably damaged the touch elements too, which would account for the unpredictable touch response.
Still, try the other suggestions. You might get lucky.