Firstly, I am sorry you are hurt and I do understand why you are, fully. I don't want anything I say next to come across like I am belittling your feelings, which are real and valid.
BUT, I am a woman who has close female friendships that are very important to me, and have been throughout my life. I have been a bride twice (late 40s now), but never once been a bridesmaid.
There have been women I half expected to ask me, but they didn't. It could be because the friendships meant/mean more to me than to them, but I choose not to see it that way.
It's a particular role, and you pick particular kinds of people to fulfil it. And clearly I have just never seemed the bridesmaid type. I'm not especially into hair, beauty, makeup etc - so that could be why. (I like to look nice, but my own version of nice - I guess I am not a conventional dresser in that sense).
I had a bridesmaid for my first wedding, an amazing friend I am still extremely close to now, over 20 years on. We holiday together etc - to be honest I think of her as (one of my) non-biological sisters.
When she got married about 6 months after me, she chose a different bridesmaid. A woman who I believe she no longer sees at all.
Was I hurt? Slightly.
Did I consider myself friend-dumped? No. Has it altered the way I see her, or the friendship? Also, no.
I get that your situation feels worse because of the other people involved, and what must feel like deliberate secrecy about it.
But there are reasons I can think of even from just reading your thread. Your son being the big one. I presume he is still pretty little? The bride and her friends may have assumed you would be way too caught up with motherhood to be able to give headspace to bridesmaid duties, dress fittings etc. And in a really pragmatic sense, they may have assumed you would need to be holding/tending to your son during the ceremony, which would make bridesmaiding awkward if not impossible.
You also don't mention finances, but could that be a consideration? With a dependant you have financial considerations others don't. There is a financial burden to being a bridesmaid, even if you don't pay for the dress itself - travel costs to dress fittings and so on, possibly the cost of appropriate underwear if it's a certain type of dress. Plus the need to find babysitters every time you attended a fitting, or bring your son, which wouldn't be much fun for him.
If your friends are lovely in other ways, I honestly don't think it's worth assuming the worst. If your friends have been keeping their distance a little since your son's birth, that too may be for the best of intentions - not wanting to crowd you when they assume you will be busy with the little one.
If you actually have scope to socialise more (not sure if your son has another involved parent? Or you may have parents of your own who help a lot with childcare) then I would perhaps organise a couple of meets yourself, so they realise you are still in a position to do nights out.
I really hope I am right, and that they aren't just a bunch of mean cows. If they are, I apologise for wasting your time with this super-long and irrelevant post.
But I always think it's better to believe the best of people, when the best is still plausibly the truth....
Good luck.