I have had that ability all my life. I do not experience any sort of auditory or visual hallucinations as a result (I can distinguish the difference between a sound or image from my mind and one from my eyes or ears). I guess it was alarming to you because it turned up suddenly and you had no prior expectation of it. Maybe for some people this is something to worry about, but as long as you can perceive the difference between external inputs and internal ones, this abiility is actually very useful.