Please help me why am i being charged?

my bank have charged me �48.00 overdraft fees for last month but i didn't even go overdrawn i have checked but it says on my bank statement 27 May 2011 CHG UNARRANGED OD FEE , 06MAY - �48.00 but i never went overdrawn can they do this? what can i do about it is there a number i can call? please help me im not very well off i so i need every penny i can get right now :-( so this do sent help