I agree there isn't some law saying you have to be utterly sang-froid about discussing things with all and sundry.
I mean, it's a snap-shot into your psyche, isn't it?
Your mother "owns" her experience in one way, and deals with her relationship with it, and other people, in her own way.
You have your own, different, relationship with it. It's impacted on your life in its own way.
I don't think you have to discuss how you feel about it, how it's affected you, with someone you don't know very well, at you birthday do!
I suspect that what is at the base of your reaction is that you're picking up this woman over-stepping the boundary of how intimate she is with you. You probably have areas of your psyche/life that are public, areas that are less public, and areas that are intimate, and only people who have shown you can trust them with intimacy have access to you.
This person quite probably hasn't earned her stripes yet, and is being a little presumptuous, trotting into the inner spaces, maybe?