I see your age is pretty young and I am pretty sure this is just a period.
To an extreme muslim you should be either treated or punished because of this fetish. But to a mild/modern muslim this fetish is normal as you cant control your desires. However this can get pretty emberessing and humiliating. Therefore you may wanna consider keeping this secret at least for a while. Make more researches about it. Maybe you will end up making falling into another fetish and just switch. Maybe you get bored of the same things and just look for a nice bubble butts :)
Anyways, what I sugges is to keep it down low for these ages as most families nor societies can handle this kinda desire.