Weird energy from a coworker - not sure what it means

I have a coworker who is a long-timer at the job. She is highly competent but is often openly negative toward the efforts of others, including mine. Her comments are often generic, but it's clear who and what she is talking about. She is otherwise professionally pleasant, and I treat her the same, but I do not trust being friendly or transparent with her beyond that. I think she has some hangups about things like not going to college (which she has dismissed without prompting as unnecessary to the role, knowing I have an advanced professional degree that many leaders in our field have), and her strong regional accent which she attempts to mask but pops up out of nowhere when she talks for a long time, which she does often. She has only recently begun being proactively friendly and playful with me (the energy I share with the rest of the team, including our supervisor), which coincided with a recent promotion she received - at first I thought she lacked social skills but I now see she had them all along but withheld them from me until she got a role above mine. She does not loom large in my professional life but I don't know how to feel about or deal with her. She seems like an ok person, but she activates my danger signals.