I'm thinking Rhonda is probably on target, check the needle position.
It's good to find out early how small differences can add up. Each seam is twice the issue. A quilt can very quickly get larger or smaller than planned.
There are all sorts of guides/feet you can get to help. For a long time I used one of my son's fancy bandages (it had stars and stuff on it), I took a piece of graph paper and stuck the needle on the marking, made it straight and then put the bandage on the machine at the marking, about 2 inches down from the needle, the little pad of the bandaid helped guide/keep the fabric in place.