We had a lot of problems along these lines when we first moved to this house. We rent, so we're not sure how the plumbing is. But one month we'd pay $40, then $13, then $30, and it was constantly going up and down with no changes in our behavior, and we knew a leak wouldn't come and go like that. We don't have a dishwasher and our washing machine was broken. We were doing laundry DAILY at our neighbor's house, and his bill was $13 every month. Then one month, his was $55! And ours was like $12.50. It was ridiculous!
A couple of months ago, the entire water company quit, was fired, retired, etc, and it changed administration and location. Now our bill is $13-15 every month. Seemed fishy to me.
I hope you get your situation figured out!