There are three possibilities

There are three possibilities about the curving:

1) You did not loosen up your foundation stitches enough given your looser stitch tension;

2) Your stitch tension is changing and becoming looser as you work on your piece;

3) Your stitch count is falling off.

It could be any, all or a combination of these three things.

Also, if you are looking at your piece with just a couple of rows completed, I find that my piece appears to be curved and that appearance diminishes the more rows I work.