Had this with a 2mo Liebling. He was fine the same day, but within 48hrs was suffering with whiplash and had a bruised penis from the car seat buckle. That was through a cloth nappy too!

We also had behavioural stuff, like fearing a carseat and pain stopping him from going to sleep alone. When the pain stopped, it was then a learned behaviour. But he outgrew it 4 years later, and it may have set in anyway.

Carseatwise, I used a carrier for 2 weeks, then did short trips with the carseat on the pushchair to get him used to it for small trips. That worked well and he could cope with the car again after a month or so.