M.F.
I've heard that the way young babies sit in the carrier can put pressure on their tummy. It seems like this is why it's recommended for preemies to ride in a car bed (a seat that looks like a small bassinet where the baby lies flat on his/her back). Maybe make sure you're not putting her in right after a meal, make sure she's burped well too, and check the angle of the car seat. You may need to roll a towel under the base of the car seat to make sure it's at the right angle.
Also, as far as testing out other car seats, Babies R Us is great about letting you pull things off the shelf and mess with them. At Wal-mart, they would not take display models down for us.