I get teary easily.
It's not usually about nothing at all, but it can be about very minor things. I know they're minor: I can tell the difference between a small pain and a large one, or varying levels of disappointment or unhappiness. But seeing the difference doesn't enable me to not cry; it just enables me to (sometimes) blink back the tears, or tell someone "it's no big deal," and continue the conversation, if they're willing.
The people I'm close to know this, and are usually willing and able to follow my lead and not have the tears derail the conversation. (Yes, sometimes I am very upset and need to be comforted and/or to change the subject my family and friends, and I, can tell the difference.)
From that angle, I would advise doing your best to follow your MIL's lead. Depending on how close you are, you might say something like "I've noticed that you seem to cry more easily than I do, and don't treat it like a big deal. Am I right that it's not a big deal for you?" That could be an opening for her to talk about it (whether she's like me, or has become seriously depressed, or something else), but she could also say "that's right" or "I'd rather not talk about myself" and bean-dip.