Why won't the afgahn i am crocheting lay flat?

i am crocheting a afghan. i have gotten to the fourth row, and it just won't lay straight or flat. it twists, and will not even lay straight when i pull on the ends. it lays like this...~~~. i am using the standard stitch. any ideas on how i can make it lay flat? oh...the very first row is a little tighter than the rest. i don't believe i missed any stitches.