If your normally happy in the car seat baby gets really fussy and upset all of the sudden one day, check their shirt. After six months of your baby wearing mostly onesies, it might just be that your old putting the baby in the car seat technique is causing the shirt to ride up in the back. Try it sometime, you won’t like it either. My daughter was always fine in the car seat until one day she just starts freaking out. While I’m checking to see if she’s sitting on a toy (another thing to check for) I happen to notice that her shirt is bunched up behind her shoulders. I pull it down, and Bam!, she’s fine.