3 year old complaining lower back hurts in car seat

[Herewegoagain2]

Our 3 year old complains on every journey, after about 5-10 mins that her lower back hurts. She has a Britax Prince car seat, can any one recommend one that may be more comfortable or perhaps padded?
I have tried tucking a small cushion behind her lower back but know this will reduce the safety of the seat so not an option and also didn’t seem to help.
Maybe a trip to try a different seat would actually be best but any recommendations or advice or anyone else’s child complained?

There was a article about a child complaining it said there was a nob of plastic on the seat sticking into the child's back.
Worth looking into , as if it's ever time, there has to be something in it.

Thank you, I will take a look, she sits with her hands behind her back crying but as soon as we are out she seems fine. She sat in the cinema on a hard booster the other day with no complaints.