Help me understand my ISFJ colleague?
When I (30F) joined our small team at work, she (37F) was super friendly to me, heard I liked 30 Rock so got me a Tina Fey's book, asked me all kinds of questions about myself, and really seemed upbeat about everything I would say or do. We became closer and I started dogsitting for her when she would go on holiday with her partner. I would often offer it myself because she was hinting at wanting me to dogsit but wouldn't ask for it directly. She even referred to her guest room as my room.
Then I started noticing weird things. She was constantly complaining about our boss behind her back and going on the same loops about the boss, how the boss is disorganized, and a backstabbing snake, and so on. But to the boss, she would be super friendly and a bit ... butt licking, even. Then, she also tried subtly undermining me in a project we were doing together. She would tell me I did enough for the project (even though I absolutely knew there is still a lot more to do), then she would make a bit of progress and proudly go show it to the boss (who acknowledged several times that I lead the project and contributed far more than my colleague). She also kept "forgetting" to give me access to the Coursera account with courses we both had to follow for the project. I got icked out about this and how she acted toward the boss in general, so i started distancing myself a bit. I would still offer to dogsit for her and was generally nice, but I didn't want to have much of a personal connection to her anymore.
Then, one day in the office she was gossiping about another colleague and saying how she should go work as a janitor because she can't remember five things at once (the colleague is intelligent and competent but has ADHD and can be all over the place). I didn't join in on her complaining but a few other colleagues did. And then she just... turned nasty toward me. She would mock how I speak and stutter, and how I look. I had it at that point and told her that her behavior is rude and insensitive. She then started crying and getting defensive and said that she would never do that (she just did???) and went to the manager to frame herself as an innocent victim who just made a joke. After that, I just completely distanced myself from her and still have trouble understanding what is up with her. Every now and then she says that she can help me with my presentations, for which I did the whole work myself and just... have more knowledge on.
If anyone has any insight, I would appreciate it. It is difficult to reconcile the person I see now with someone who seemed so warm and caring at first. Sometimes I used to feel some bittersweet sadness for her when she would read me her horoscope for long stretches and thought how she must not have many people listening to her and noticing her inner world.