I showed your post to my husband and had to emphasise I hadn't actually written it myself!
I got slapped in the face with a flat hand the other day. He's usually amazing with the cat but will hit her or try to yank her tail from time to time. Today we've had repeated strops/ tantrums for any number of reasons (me wanting to put him in his highchair, him pushing the pushchair and getting stuck, him "only" being allowed 6 large spoonfuls of chocolate cake, me not topping his cup up when it's not even empty etc). Nappy changes are a bloody nightmare, he hates lying on his back. He has also tried to bite me a few times, but I've seen it coming and managed to dodge it.
He's 22 months and I'm 11 weeks pregnant. Exhausting is the word indeed.
I don't think the violence is anything to be particularly concerned about at this point, but I would make it very clear that it is not OK. I grab his hand when he hits me, look him in the eye and say it's not allowed and is naughty. Other times I hold my index finger up and say stop. If he continues, I put him on the floor/ in his cot/ any safe place that is not where he was kicking off and leave him for a few seconds.
The tantrums are because their brains haven't developed enough yet for them to snap out of it. Distraction is the only thing that works. I agree with a PP that he's probably frustrated at his inability to communicate. DS says "more!" all the time, it can mean anything from "I'm hungry" to "I want that toy/ I want to go outside". Could you contact your health visitor for some support, even if just emotional/ moral?
Good luck, and try not to worry!