Originally Posted by Sadiemae
The only thing I can come up with is that one of the fabrics stretched.
I had this problem once a long time ago, except that one of the fabrics shrunk. One of the fabrics had polyester in it (remember this was a loooong time ago - I think around 1973) and every time I pressed it, it shrunk.
I had already pieced a block and when I pressed the completed block, POOF - the center square shrunk. Puzzled, I figured I hadn't marked the square correctly. I unpieced the block, removed the center square, recut, repieced (same fabric)and .... you guessed it! PRESSED it again. POOF - center square shrunk - AGAIN. Hmmmmm....
Got smart this time! Cut a center square out of the offending fabric and pressed it FIRST this time. It shrunk about 3/8". Threw out the fabric and was very careful to NEVER buy fabric with polyester in it again. Remember this was around 1973 and polyester/cotton blends were rampant. We are so lucky to have so many wonderful quilt shops with 100% cotton fabric!!