After I iron my fabric, I fold it selvage to selvage and let it hang freely. You can see by whether it hangs straight or is pulled toward one direction. If it is pulled, then I carefully shuffle it until it hangs straight.
I then lay it on the cutting mat and smooth it out with my hands, but I do not iron it.
It sounds like you might not have had the two halves aligned and the ironing emphasized that distortion.