I suspect the problem is the edges of the extension table. You would likely benefit from making a styrofoam surround so that your quilt is lying on a much larger flat surface. My dh made me one and it helped a lot. There are directions on Youtube:
https://www.youtube.com/watch?v=g14g...5C338341F28BF9
Also, the fact that your neck and shoulders are in a knot indicate to me that the surface you are working on is too high. I found that placing my sewing machine (along with its styrofoam surround) on my cutting table and quilting while standing up took all the stress off my shoulders and neck.