I'm so sorry this has happened to you.
Please know that it isn't aceptable for your DP to ever touch you like that. There is no situation where you can shrug it off, but certainly not after 6 weeks of having a baby; your body is still healing.
You say your DP is lovely and kind, but you also say it's not the first time he's pushed you. It may well be due to the stress/lack of sleep that comes with a newborn, but that doesn't make it okay. How do you know he won't shake your DC when he's stressed next?
I would leave if my husband laid a hand on me, especially if I had a child that could be at the mercy of one of their rages. However, before I made such a move, I'd want to know why. Has something happened at work? Has he been made redundant? In debt? Is he depressed? Worried about his health? Found a lump?
That's not to say any of those scenarios make it acceptable, of course! But it would make the difference between me leaving and never looking back, or leaving but demand he sees a counsellor/doctor/get anger management with view of trying again when he gets his shit together. It'd have to be something extreme for me to consider staying, and even then it certainly wouldn't be under the same roof. You and your child come first.
Lastly... makes no difference if you're pushed to the floor, bed, wardrobe, roundabout (okay, I'm speaking from personal experience here on the last two) - you deserve better, and so does your DC. x