Heh rage away babe
That is really intrusive and presumptious of her, yes, who knows why she felt the need to say that, maybe she misunderstood something you said? To be honest I don't even understand what she said that much, was she saying if you go with your baby then the other lady will go with hers? Or is the other lady already going? What's it to her if there are babies around any way, your baby goes where you go after they are here, simple as.
I don't know what 'event' this is but I don't see any issue with taking a nine month old, especially if both you and OH will be there so you can both have time to enjoy the event and be with baby
Me and my partner are thinking of booking a camping trip just two months after my due date, camping is our thing, loads of other parents will be there that we know and besides we can always cancel if I'm not feeling up to it!
I assume you can cancel if at the time it's not right any more?
Take a nice warm bubble bath