Thank you to everyone who's commented, whether sympathetic or not. I really wasn't looking for sympathy (I wouldn't have posted on AIBU if I was to be honest!) and I actually have found it helpful to be told I'm BU overall as it helps to give perspective. I think as some PP have mentioned, a 3 and a half year old suddenly seems very grown up when you have a baby as well, and I know I've probably been expecting too much of my eldest child on occasion. Although I'm certain (in fact I know for sure!) that he definitely tries to annoy me deliberately sometimes!
This has been the hardest 6 months of my life, dealing with the questions and uncertainty around my DS, who although I did worry about having ASD when he was a baby, I pushed those worries to one side when he started pointing etc just after his first birthday, and pushed them further away when his speech developed well. This is what I meant when I said I thought he was "developing beautifully until his playgroup flagged some behaviours" - he's always been a bit "quirky", for want of a better word, but due to his communication and reaching his other milestones, I really didn't think I needed to worry about ASD any more. So it has come out of the blue, and is made so much worse by the fact that it's what I've been worrying about in the back of my mind all along.
Apologies if I have offended anyone with children with ASD. I know it doesn't have to mean the end of the world, and I also know that if my DS does have it, I am lucky in that he communicates/is potty trained/sleeps etc and is interested in people and children, so would be viewed as "high functioning" so to speak. If he does, of course we will move on and learn to live with a new "normal". But I can't pretend I hope and pray he doesn't have it; as let's be frank life can be tough enough for NT people as it is.
I took the advice on this post and when DS started copying me making milk for baby DC again, I asked him what he was going to do once she'd finished her milk, and he told me park his car in the drive, proceeded to get on his ride along car and tried to put the poor baby in the boot
she didn't fit so he gave up and later made me some pretend porridge which was lovely and we had enjoyed bonding over that.
His pretend play is something I've worried about as it rarely seems inventive and tends to be mainly limited/based on recent every day events eg going to the shop/farm etc, but I don't really know what the range of "normal" is for a 3 year old. It's difficult to not really know what you're supposed to be looking for when it comes to "flags"...