If the fabric is flannel (that's what it looks like on my computer) your seams weren't deep enough. When I sew with flannel my seams are usually 1/3 to 1/2 inch deep and I use more stitches per inch. Loose weave fabric will do that. My first patchwork quilt I pieced by hand similar seam separation. They weren't flannel, it was a kit I bought at Woolworths that were pre-cut squares. Some fabric was nice and some not so great.