Same story as OP's nearly. Had first baby, bad 2nd degree tear (took forever to sew up after), i suggested sex at around 4 weeks to husband and that he would have to be really careful and really slow. So we start, its uncomfortable, it then becomes pain, I asked him to stop at least twice as it was hurting, he continued for 30 second or so afterwards or so it felt like. I was quietly crying. Can't remember much more of the details it was 13 years ago. But I do remember the hurt that he would do that, I remember the utterly broken trust between us. I remember I had said stop and he had carried on. After when eventually when we started having sex again (when I was well enough and thought myself over the trauma) he would get pissed because during foreplay/during sex I would have flash backs to what happened and want to stop. A massive inconvenience to him from his reaction. I was made to feel like shit and put up with his sulking when he didn't get more sex for a long time and I asked for reasons/apologies. His excuse at the time was he 'didn't want to go soft' so kept going. Around 5 years after that when I was making a statement to the police (he had threatened to hit and hurt me) they classified what he did as rape. As did my health visitor. As did the social worker.
At the time, he was my husband, I had just had our first child. I could not believe he could do that and tried to brush it off, as he did, repeatedly. I must say his apologies for it would have done a small child proud...he would apologise ONLY when our marriage was on the line and would do so grudgingly and clearly NOT sorry at all.
OP, I'm sorry this happened to you. You should not EVER have to ask to stop twice, three times etc no matter how close he was to coming. That is putting his pleasure over your pain and like others say, he should have been on high alert to your comfort, not his own. I used all the excuses Villageshop et al used 13 years ago to excuse my ex husbands behaviour. It does not EVER excuse it. We separated 7 years ago. Best decision ever.
I would not jump to any immediate action, but I would be watching him like a hawk now. Don't be surprised if it takes you weeks/months to have sex again. Or even a few days to 'see how things go'. It is very difficult to conceive of any action just after the birth of your first child with your partner. However, you could speak to your health visitor, speak to your GP, ring a helpline to talk through your feelings with someone anonymously, confide in a trusted friend. Just don't bury it like I did for years, living with that feeling of suppressed anger and mistrust and the ex refusing to take my feelings as truly valid.
And OP, I'm guessing you won't come back to this thread to comment. When I posted, the same as you, instead of the posts saying 'LTB' helping, they made me want to protect and defend him and his actions in my head, that he wasn't really like that etc. Try listening to your gut feeling about what happened, and talking to someone or a helpline or a counsellor in real life, you need the support to decide what to do, even if that is to brush it under the carpet til you are ready to deal with it. This is your life and whatever you chose to do is your decision xxx