You have just had a baby and your body is not used to it not having (baby) inside of it. Plus you had a surgery which means your body is getting itself back together from having had trauma of the incision(s).
When you have a baby your muscles are stretched and don't respond to your commands to move as they did. Also you do not realize how much and many times a day you use your stomach muscles to sit, stretch, move, twist and sit. Driving makes you do all these things at once and if you have never experienced a surgery prior you will not undertand until it hits you and you feel very very sore.
As other posters have mentioned just the act of sitting in a car with the stitches still in pulls on your body and it hurts.
All I can say is take it easy now after baby and REST not run all over the neighoborhood because you have just had baby and feel free. The human body needs time to heal from anything that you do to it from falling down bruising, to birth, to surgery. What's the rush you have the next 20-30 years to get there. Take care of your body and it will take care of you. I say this as such because what you do to it now will show up later down the line like 30 to 40 years.
The other S.
PS I had a hysterectomy and stayed home for the two weeks which is very similar to a C-section and was sore. Just take your time and heal then you can run a marathon and not have pain anywhere.