what mental health struggle can cause changes in mood

What could be causing my work colleague low moods. I have a work friend who every few weeks seems to be distant. We got on very well and I would say we are work friends. We've even met for a drink a few times outside work. Sometimes I feel very connected to her but every few weeks or month there is a period of time where she feels distant. I've asked her if everything is okay and she always she she is fine. I've even brought up the fact that she seems more distant. She always says she is not. I realize I may be projecting but I don't think I am. She has alluded to having mental health struggles before but not depression (as I spoke about my brother who suffers from serious depressive episodes and she mentioned never experiencing it). I know no one can diagnose her here nor know what is going on for her but guess I'm just trying to understand her more as it is triggering alot of my old wounds. What mental health disorders can cause someone who seems to be empathetic, caring and kind most of the time to then be distant, more emotionally unavailable and blunt at other times. Also When I mention that she seems different she adamantly denies any difference so it's like she doesn't really see it.

I should add that I've no intention or bringing this up with my friend or ever attempting to 'diagnose' them as clearly that is not something I can do it's more just for my own understanding as I am so confused and triggered by the relationship/friendship. Or if anyone has a similar situation with their friend or loved one that could provide me with some insight. I find things so much easier when I can understand reasons for things or even potential reasons

Without knowing more about her personal or home life, I cannot say. It may be situational or organic.
If she has alluded to past problems, at least she has already recognized them as issues?
Maybe part of it is your perception or expectations on how she should be or how she should conduct herself?
I wouldn't try to change her, unless she asks for your help. Proceed with caution. Respect who she is at work for now.