Yeah, it's possible to suddenly crash into a depression.

I agree with Christina, though, that not all actions have a mental illness component, so it could be anything. Bizi is also right that she doesn't sound interested in finding out, which I know makes things difficult for you.

Try not to pressure her into talking about it if she doesn't want to. Remember to respect her boundaries. If she wants to open up and ask for your support, then by all means, talk to her and offer her your support. But from my POV, it sounds like she is confused and overwhelmed -- possibly embarrassed, even -- hence the distancing and passive aggressiveness.