Losing more IP than I should when buying a Champion? :|

Hey, I just bought Alistar on my account (Whooooo support main!)

After I bought him, I went from 5200 IP to 2500 IP. I didn't buy anything else before hand and I'm pretty sure you can't buy a champion twice. math pls ;-;

Is anyone else having this problem or is it just me? Is it a bug? Oh, and can I get the ip I lost back too? ;-;

Edit: Nevermind, after winning a game with Alistar (^.^) it seems I'm back to 3900 again.