Tinytink, you are not being unreasonable, over emotional etc. The comment your mil made was not nice. This is alpha female behaviour designed to establish her as the dominant female. Not OK behaviour.
As so many discussions on GN are about, when our offspring have their own families it is time for us oldies to take a step back. We have had our time running our own families and it is time to graciously and lovingly hand over to the next generation - offering our support if needed and requested. If we have done our job properly then we should be very confident that our adult children are well and truly able to manage their own families themselves. (And I do understand that in some particular situations that is not always the case.)
If such a comment is made again I would suggest saying, very politely, that it was a hurtful and derogatory comment and if she can't say anything nice then don't say anything at all. I would add that you are happy to have her support and assistance provided that she understands that the baby is yours and your husbands, not her baby. Any thoughts and contributions she may wish to make will be taken into consideration and under advisement.
You need to have some honest and open discussions with your husband about this issue and about boundary setting. You do need to be both singing from the same song book.
Good luck Tinytink, I think you are going to need it. ?