You might have stretched the pieces when you pressed? Did you possibly "iron" instead of pressing, and push the pieces around and out of square?
I press the seams in one direction on the first and all subsequent odd rows and press the seams in the opposite direction on the second and all subsequent even rows.
That way, when you put row 2 on row 1, the seams should nestle. If you pin the seams, then you can adjust (fudge) the fabric to fit.
Don't worry too much. It's one of those things every beginning quilter has to get through. Shoot, sometimes, it still happens.