Ashes,

I had a bunch of plants on a windowsill do the same thing recently, because they're in a South window. Over a short period of time the light conditions went from being bright shade to full sun. The things I had in humidity controlled environments were partiucularly unhappy. Have your light levels changed recently?

I grow mine in full sun outside, but when the temps get cooler they tend to do that. Mine is starting to just send out some new growth now.