You're not experiencing pp rage. Your MIL had straight up audacity to make assumptions. Of course you're not ok with her taking him on trips without you. Doesnt matter how 'nice'or 'good' she is. You lack boundaries with your husband's parents and I don't doubt your husband lacks boundaries with her also. Boundaries are not walls, they tell people how to treat you and your limits when it comes to your kids. Strating to sound like she's starting to take advantage of the grandparents privileges because you've allowed it. You don't owe her time with her grandchildren let a lot an interstate trip with your infant son. That generation would pack their kids on flights to their grandparents place so they could party. That generation did not enjoy parenting as much as they could have and it's not up to the younger generations to fix thst for them. You sound like you're not willing or maybe scared to tell her no? What did your husband say about it?
She might think she's doing you a favor or that it's totally normal for a baby to be independent of his mom at that age and not understand how you feel about it. What you describe doesn't sound like pp rage at all but just a normal reaction to the thought of separation from your baby at this age. You don't mention what your husband thinks about this at all? This is his mom and he should really be the one explaining to her that you guys don't want to send your baby across the country without you!
Just remember that no is a complete sentence and that her feelings are not your responsibility! You're still offering to take the trip as long as you're there too which sounds like a great compromise and if she doesn't like that then he doesn't need to go up there at all!