display name problem

so this is the most related place i could post this question...

ok so i had this problem for a few weeks now...
i used to be able to change my display names but one day it suddenly went
gizmo and i can't change it anymore...
when i'm online i can change my display name to anything i want but after
i'm done with the day, turn off the computer and come back on the following
day, it goes back to the one before it got changed (meaning i've had the same
display name for the whole time i've had this problem)
i've tried goin to option -> sign in -> and unchecking "use the same display
picture...." but this always seems to be checked again whenever i sign in
again.
even when i sign in from another computer my name stays the same and i can't
seem to change me
HELP ME PLZ I"M SICK OF MY DISPLAY NAME XD
thank you