Strange Lovebird Tail-shaking Behavior

[KNG]

Hi everyone, the last few weeks my peach-face lovebird has been acting strange (she is housed with a fischer's lovebird, also a female I think). She has been shaking her tail feathers constantly. It reminds me of mating behavior in female finches, or just shaking the feathers after preening/a bath. I don't believe this is mating behavior because she does it constantly, and she's never done it before. These birds are rescues so I have no idea how old they are, and they aren't tame either. Any ideas of what could be wrong? No eggs have been laid recently (they don't have a nest, but one, or maybe both, still lay eggs from time to time).