Do you receive the water bills in the mail? Those bills hopefully would include what you paid last month + what you owe for this month, usually in the portion you don't need to take in to pay the bill with. No matter how disorganized you are, you should try your best to keep those someplace you know where they are, so when situations like this come up, you can go back a month or two and point out that "Yes I did pay those months."
my BF has a little mail organizer he puts all the bills in. then at the end of the month he sits down, pulls out the check binder and gets the bills ready for mailing. except for the water bill, cos sometimes he just drops that directly in their locked payment box. and rent - often that is put in the office manager's hand directly or in the little lock box outside her door.
then he saves the bills and puts them in a single envelope and marks it for the month. he usually keeps a month or two on hand just in case something comes up where he needs the information