Why is my friend/flatmate friendly one day and avoidant on another?

Anonymous
This is about my f23 flatmate and reasonably good friend (or so I thought), and me (m25). We got along well initially. Maybe some romantic feelings from her side at some point but not sure and probably not relevant here. Anyway, at some point things turned a little tense between us and I don't know why. By now, we alternate between good days and bad days.

On the bad days, she seems to be avoiding me. Doesn't say hello when I know she has seen me or doesn't say it back when I greet her. Acts indifferent towards me or what I have to say. Overall I get the impression that she is either scared of me (but there is no reason I could think of) or just straight up rude (which is definitely not how I know her, normally she is very polite with everyone). She often goes out of her way to make everyone feel included but apparently I have ticked her off enough to sometimes do the opposite to me. I also wouldn't say she's a shy person from what I've seen in her interactions with other people.

Then we have the good days which happen equally as often as the bad days. She's friendlier, more cheerful and makes an effort to talk to me. Asks personal questions, especially about my future. Once told me after I had been away on vacation for a couple of days that it had felt like a long time to her that I was gone. Sends me pictures and funny videos. I noticed that she often comes out of her shell when we are in a group and then she'll be talk me more easily. One and one talking is generally more difficult because I get more distant vibes from her then.

What could be the reason (s) for this confusing behavior? And how do I deal with it and improve our time together? She's someone I care about a lot but the constant mood changes make it difficult to interact with her normally on a daily basis. Which would be a pity because our friendship is important to me.
Why is my friend/flatmate friendly one day and avoidant on another?
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