You've likely been walking great distances or something that would cause your butt to be worked out. I remember when I went to hawaii that after 5 days (was there for a total of 9 days) a swim suit that barely fit (struggled to get on) on the first day because my ass was too big (assumed to be fat) now could fit much more easily (fat went away) but when I got back from hawaii, I was very sore on my ass. It seems that I've kept my butt fat off because I tried the swimsuit six months later and it's the exact same thing (fits much easier).