Help i am felling guiltty...?

ok i need some opinoins because i fell realy guilty right now because i slaped a girl at school. so why i did it well becuase she was bothering me at lunch time and she was throwing apples at me. than she walks up to me like nothing and asks me if i am gonna drink my water and i said yea (in nice words) and she takes my damn water and drinks it all so i say wtf she says it was just water and throws it at me so i got mad and slaped her and she takes of running crying. and now i fell bad was that a bad thing i did i mean i slaped her not punched her. please advice.