Congratulations on your new GC.
From my anecdotal experience I’ve not heard of any of my friends or their dils/dd’s having late onset PND at 20 months but I’m sure it happens.
What I do see reading through the lines, in this statement
Then last year...she just changed towards me. Now I can do nothing right. She will verbally attack me and accuse me of all kinds of rubbish for no reason!
is that maybe an alternate reality is happening to your version. Maybe DIL got on with you before the baby arrives, but after the arrival, be it unwelcome advice, ‘help’, anything that could be viewed as overbearing to a new mum and then at some point she snapped, she’d had enough of being polite and starting pushing back.
I only know this because although it wasn’t dils pregnancy that kicked off my DILS issue with me, I was very surprised when she suddenly withdrew and pulled herself from family time/events.
You would need to give us more information, what dil said about you that you describe as ‘rubbish’ to build a picture of what’s happening.
Don’t talk to your son, don’t mention PND (this will wind up your dil no end if it gets back to her, and it’s not PND and she’s just annoyed with you as her MIL), I would give her space, and the next time she gets annoyed request having a chat at better time as you feel you might be stepping on mums toes.