She got mad, said "I thought I was number one with you." then proceeded to start to tell me some gossip. She never acknowledged the person I introduced her to. I walked away with the other person.
This being angry at you for walking with someone else is a

. The two of you weren't best friends for years, you were aquaintances in the park, and she says this.
Give yourself some credit- sometimes we don't know until we know. There is nothing wrong with meeting someone and being empathetic to their story. I think another

here is when she started speaking of very personal information if the two of you didn't know each other well, indicating poor boundaries. But all you did was listen to her.
I had something similar happen to me a while back. Someone I know not extremely well began sharing shocking information. I learned from others she would say the same thing to them, in situations where it wasn't really appropriate. Someone else who knew her better said they thought she had BPD.
Sometimes it's not possible to know at first that someone is disordered. People with PD's can hold it together at first, and it may be that later we get the sense that something is off about them. If you were to not speak to anyone- or ever meet someone- that would be a boundary that closed you off. A boundary can be semi-open- meet people, but take your time with them. I think where we wavered was listening a bit longer- due to our own family patterns and being empathetic. I thought this person who was saying TMI was odd- but odd is different from toxic. The TMI is the poor boundaries in sharing information. That may not be an issue if it were only with someone we see once in a while, in a public setting- we'd shrug it off as "well she's a bit odd" and keep a relationship distance from that person- cordial but not more.
Where this stepped into disorder is when she said that remark about being number one. That you recognized this is a boundary. We are going to run into people who are disordered at times, and a clue for ourselves is our own discomfort. We may take a longer time recognizing this due to our own family dynamics but - you did and now you know- this person - there is something "off" about her.