In order for us to answer that, we would have to know what she was doing for the past 3 months and if there were changes to her exercise program (assuming that something in the exercise program is a cause for this). Also, what was her eating habits like?
Aside from measuring her body fat and weight, are there any other anthropometric measurements that you take (such as skinfolds, waist circumference, circumference on other body parts)? The less tools that’s used, the less accurate a picture you have of what’s going on with your client.
It could very well be the device itself or even the conditions that you measured your client. Normally when you take these measurements, they should be taken under the same conditions as the first time you measured your clients. Did you make sure the conditions were the same the last two times you measured your client with the device?