Minecraft PE? MC PE?

I just bought Minecraft PE and I had to make an xbox account I made it with gamer tag and when I went back to Minecraft app it said my username. But when I closed it and opened it after some six hours my tag above the player is different.

I double checked my xbox account it says my right gamertag but in Minecraft it says its something random. I actually did not change it. I never tried to change it, ever.

I then went back to my xbox account thinking if I reset the tag it will reset in my Minecraft but when I tried it said I had used up my free gamer tag change.

The problem is I never even tried to change it before. I have no idea what happened or what to do. If anyone else experienced this, please help.