Yes I get this often and always have, with any strong mix of kava, fresh or dry.
I think part of it is just that you are so relaxed that you breathe shallow anyway, but occasionally I feel like I need to fill my lungs to ease a slight discomfort, and take a deep breath. I have the same the next morning after a big session also. Its never really bothered me, and I never really gave it much thought until you mentioned it - but, yes, its definitely an effect. It was worse in the old days when I used to have a smoke after every shell.
Still, we must be breathing enough or we wouldn't have the energy to mix up another bowl. 5pm and time for mine now...