I'm a contractor. I billed a client for machine rentals. They paid. Can they dispute the payment 6 months later?

I've just finished my first large job as a company. It was a learning process. I billed the customer weekly to bi-weekly for majority of the project. I billed them on 3 or 4 occasions for cost of equipment and operation. The last payment being recieved beginning of October. It is now March, after receiving their final bill, they are trying to dispute the rental charges from 5 months ago