The reason you are losing PP usage is because your opponent has the ability pressure. Pressure raises the Pokemon's PP usage. With this said, it's not going to increase your maximum PP, it's going to decrease the amount of times you use a move. Fortunately, this only works sometimes - not always.
For example, I was versing a legendary (with pressure) with my camerupt having the move eruption. Every time I used the move, I would lose 2 PP rather than 1 PP.
Hopefully this Helped!