If her kid plays nice with yours at the party and other times, I'd let it go. There are times we don't invite the neighbor for various reasons, or go ask her to play. Like, maybe her cousins are over or it's a holiday or maybe they just wanted a playdate with a few kids. Or maybe I know we need to go at a certain time and it wouldn't give the kids time to play. Etc.
She might not like you, but then again it might be something simple. I think that as hard as it is, sometimes you just need to shrug it off. Not every kid will get every invite, not every neighbor will be your friend, not every parent of your kids' friends will be someone you also like. It can sting, don't get me wrong, but I'd just invite someone else over to play and not worry about it. If your son asks, say that they must've just wanted a smaller group. Maybe they figured you'd be getting ready for the party.
Just a bit ago I sent an email to a subset of friends asking for a playdate tomorrow. Anyone I left off was left off because I just wanted a few kids, we haven't seen some of them in a while, etc. I'll see most of the other kids/moms at other events next week.