Missing credit on my account.

Hi there. Before i redeem my itunes card worth of $25 the remainning balance was only $0.21 so i decided to redeem my itunes card $25 when i checked my account it says tha i have a balance of $25.21 i download paid apps worth of 0.99 cut the rope 2 and the other apps worth 0.99 as well then i checked the remaining balance is $23.23 and then after that i decide to purchase a chips from social tongits worth of $4.99 but when i checked the remaining balance its only $17.82. What happened ?