You have found a wonderful "secret" to machine quilting. I couldn't figure out why my back puckered the first time I tried machine quilting (35 years ago--doing a baby blanket for my youngest child). I bought a walking foot 16 years later (because I couldn't get velvet to feed right through my machine and I was doing a prom dress.) I soon tried it for quilting and from then on, no more puckers on my quilts. Wonderful.
Your quilt is cute, too.