Hi, sorry I think this was a long time ago you wrote this.
Did you find that your hypothesis above was correct? Iíve had this problem a long time and have just been putting up with it and applying lypsil or other lip balms twice per day to cover the problem such that it exfoliates after Iíve brushed my teeth.
Fungal infection sounds possible, and perhaps electric toothbrush could have been the cause... Anyway, did you cure yourself through your treatment method.