Hi there. What vehicle do you have, and what seating position is she in?
Just to be clear, it's the vehicle seatbelt that's going over her, right? Not a harness built into the car seat?
Are you locking the seatbelt by pulling it out all the way and then letting it retract, like you would when installing a harnessed seat? If so, that's probably the issue. The belt shouldn't be locked.
Is it just the lap portion that's too tight? If so, you might have belts that lock at the latchplate, and that could be getting too tight if the shoulder belt is pulled up.
We usually recommend that children stay in harnessed seats until they're at least 40 lbs and at least 4 years old. If you have the ability to put her back in a harnessed seat, that might be a better option for now.