Sewing on my fourth square, I noticed it was stretched a little longer at the corners whereas the other two seams much less so. Turns out I had my first three squares all aligned the same way of stretchiness by sheer luck, whereas this last block was not. I have no idea why this is happening; the fabric is quilt store quality, made by Kaufmann. I also did prewash it. The machine I'm using is a c. early 1940's Featherweight and it was fully serviced at the end of last year so I don't think it's the bobbin tension. I've set my needle tension at 3.5. For now I'll just take this seam out and redo it with the fabric aligned the same way but I'm lost as to why this is happening because it really doesn't seem to be an issue for others.