FWIW, I've found that I feel very stiff when I get back to yoga after a short break, but the results of the previous months of practice aren't gone -- it takes a few sessions for my body to loosen up again, but then I'm back where I was.

This - there's also the potential issue of perception, in that you may very well be nothing like as bad as you think you are, but the contrast between what you expect your body to be able to do and the immediate results winds up with one feeling like one is stiffer or tenser than one actually is.