Your scales must be bizarro!
Maybe it's time to change the battery.
Humidity can affect scales. There is often a slight difference among scales so it is best to always use the same scale and to weigh at the same time of day. Placing the sales on a hard surface is best - placing the scales on carpeting can make a difference.
Try to standardize your weight conditions - shoes off, same time of day, put the scale in the same place, wear the same thing (or not!) when you weigh.
Our weight can vary a considerable amount from morning til night. It is so weird - sometimes I can weigh as much as 4 pounds more from the time I get up until the afternoon.
I used to freak out my husband by slowly adjusting the dial upward on our scales...can't do that with the digital scale!