Charged unplanned overdraft fee for overdraft fee levied by bank.?

Please help. I went overdrawn by �5 and was charged �85 pounds. I do not know how they worked that out , but that was my problem for going overdraft. I had just �20 in my account so my balance went to -�65. Two months later, there is a charge of �75 on my account for going overdraft by �65( from the balance of �85 charges ). I paid the �65 in the account the next day. I got no letter saying the unplanned overdraft fee was due to come out. I have called and was told since I am overdrawn again, in two months I will be charged. So therefore I have to pay Lloyds TSB about �75 - �80 every month. Please help.