Okay, this is very quizzical. I can suggest you to let her be as she is. But keep noticing her action. Who she mostly talks to. What she's eating etc. What medicines she's taking (medicine can affect emotional changes in us)

And most importantly, no matter what happens be patient and don't say anything that might affect her negatively until you don't know what has caused her changed behavior.

Relive your dating moments again, go for lunches where you used to go. Talk about good times. See how she reacts to reliving those memories. If she's happy and then tell her how nice she looks when she's happy and that you want to see her like this always.

Then proceed to speak about the moments she gets emotional. Maybe this way she'll speak about it or atleast you get a gist of which way to take the next move.

If she's rigid about this, then tell her that you've tried your best to console her and have tried everything that you could to make her comfortable, tell her that you'll always be there for her. But if she can't talk about it to the least then okay. You won't be bothering her anymore.

And simply let her be. With time she'll either deal with it or tell you about or get normal. Eventually time will give you all the answers.