I'm not sure what your question is: 'AIBU re those 5 seconds?' Unreasonable how? You are not unreasonable in feeling the way you feel, but hang on, take a big breath and think of the man you love.
This is not rape as I understand it: 'All was well, it was better than expected.'
Do you love your husband, OP? Do you feel secure in his love for you? I take it that you did, up to that point, consent. Were you trying to get into it and it was only at the last minute that you thought, NO!?
This one time your beloved, in the throes of passion and love, for the first time since you gave birth, carried on. It hurt, you wished he'd stopped, he carried on for 5 seconds. In the throes of passion he didn't realise this was hurting you, he thought, if he thought at all, I can't stop. 5 seconds later, he stopped. He probably thought that was stopping in response to you asking him to stop. It is not clear from your post whether you told him it was hurting, or if you just asked him to stop.
In the scale of your whole lives together, this is a small hurt to get past. Talk to him, tell him how you feel, tell him everything that you are feeling and let him listen and learn.
Here on Mumsnet it often seems to me people live unrealistic lives and shout LTB, or rape, or abuse, at every given opportunity. Real life is not so black and white. Errors of judgement occur, and to let them jeopardise what could be a long and wonderfully happy family life is to cut off your nose to spite your face.
You know the man, we don't. Do you love him? Is he a kind man? Do you see that he meant you no harm? Men are often unaware of the physical changes to women's bodies wrought by childbirth. Teach him the reality, and if he's a good man he will understand and want to find a way to have sex that is good for you as well as him. But men need educating, in my experience.
I suspect what happed was down to him being unaware, rather than deliberately cruel or selfish. But only you know your husband, so trust your instincts.