My clients weight didn't change but her body fat went up from 31% to 37%. What could cause the increase in BF?

We measure my client once a month and her weight is steady since 3 month. Last time her BF went from 31% to 37%, we measured the following week again to make sure that it wasn't an error from the device, but it was still high. What could be the cause for the sudden high BF?