Why the wobble in my placemat?

So I tried my hand at the placemat that is quilted by going around and around until in the middle.  I had questions previously and got that resolved but not my placemat doesn't lay completely flat, but wobbles around the edges some.  What did I do wrong?  How do I fix it?  I have a quilt that I'm making and am to scared to quilt it and thought if I tried this placemat first I'd get a feel for quilting but now I really don't know if I can handle quilting.

Usually "wobbling" stems from one fabric being pulled or stretched while the other is not. This could be caused by using two different weights of fabric, different material (cotton vs a polly-mix), or one fabric pulled/stretched more while sewing the seam which can be caused by you or the machine's pressure foot gripping one fabric more than the other. 
You could try washing it in warm to sort-of shrink everything to fit better, but most times there isn't am easy solution.  Next time, be sure you're using like fabrics, pin to keep everything together, and use a walking foot or if your machine has the option, raise your pressure foot tension so it's not pushing down on your fabric too much.