I would check her car seat to make sure she's in it correctly...she could be in too tight. the straps could be too low and "cutting" into her shoulders..
It's not about being fed - it's about being comfortable...she picks up on your stress and is using your stress cues and getting upset. If you just fed her - did she get burped as well? Sorry - not trying to be mean - but sometimes we forget that burping can make us VERY uncomfortable and if you are putting her in her car seat just after she eats - she's probably got a lot of gas...
Put her in her car seat in the house - see how she handles it...then make sure that there is padding for her back and sides...make sure the straps are above or at her shoulders - NOT BELOW them...make sure the buckle isn't cutting into her belly...
My boys and the soft toys to play with - the mirror so they could see that they were not alone....once they got their vision squared away! :) It also helped when one of us with one of them in the back...