You know, It's amazing that not long after I posted this, I was having a conversation with my Dad, and when I mentioned my discomfort living with my mom (without explaining all of this) he immediately began to affirm what I had been feeling! He said he noticed that she had been pulling away from us kids and acting kind of separate from us, almost like a stranger... He even told me that his girlfriend observed my mom acting distant and strange to my brother, not asking how he was even though she hadn't seen him in a long time!
It's so eerie, but it's also relieving to know I'm not imagining things. If an outsider like my dad's girlfriend could see the oddity in her relationship with her son, then clearly something weird is going on. And not that it's only been these past few years, but I guess she's had a bit more freedom since we kids went to college, and she decided to take a step further back...
Thanks for the hopeful comments! It's good to know i'm not alone.