J.B.
I do understand where you're coming from but I personally wouldn't notice one way or the other so I can understand where the other mom is coming from in that she probably lost track.
I have 4 kids and at least once a week have a conversation with someone and have no idea who I'm talking to. These are people who know me by first name and know my kids. I don't recall us ever meeting or chatting before and yet there I am, in the midst of a conversation where they're telling me something funny about one of my kids. Last week someone complimented me on how polite my 8-year-old son was when he was at her house recently and I literally had no idea who she was, who her kid was, where she lived or that my son had been there at all (my husband had handled this one on a day I worked late).
So with that in mind, I can tell you that I have no idea whose birthday parties my kids have gone to (I do know that among the 4 of them they have gone to 11 parties in the past month alone), who they're still friends with etc. and the guest list to their own parties will be based on who they are friends with at the time and how many we can accommodate, not who we owe an invitation to.
So if I were you, i would just assume that the other parents are as flaky as I am when it comes to their kids' social lives and assume that it wasn't personal.