My extra RP points was taken!!

So I still have some hundred RP point In my account, I want to get the underworld wukong skin today and bought 5$ worth of RP point using my credit card but it was 10rp short. I then buy $10 RP point remember it all add up to around 2700rp. The underworld wukong skin cost 1350rp but when I bought the skin my account only have 22rp left!!! Why?!! Is this a bug? Riot please fix this in scared to buy RP points again.