I got a mash up that I paid for taken away!

So a while ago i bought the Chinese Mythology mash up, and i was thinking that i wanted to play a world in that mash up so when i went to make the world my mash up was gone and the game said that i need to buy it AGAIN which is outrageous because i already bought it and im not buying it again just so it can get takin away again. But i have bout other world packs around then time i bought this one and i still have those. I'm actually quite sad that i lost this mash up pack because i loved it. I have even used the mash up texture pack in normal world so it looked better and i know that im being a little stupid but i already bought it before im not doing it again because i should not have to buy it again because i ALREADY BOUGHT IT!!!