My wife gets all emotional sometimes, and when she does I say "Are you PMSing or something?" and I usually either get "I dunno but I shouldn't be" or "Yeah I guess so". She's a very rational person 99% of the time, so she recognizes when she's being irrational. Only once did I bring it up and got chewed out, but that's once over a long period of time, and we had already been fighting.
I asked her once when we were at the in-laws and her Mom's chin almost hit the ground in shock, and then further when she didn't get mad at me! lol