I'm no help. It looked alright to me.

I think if you measure your sewn pieces, each one of the 4 you will find they are different sizes. Your seam allowance may not be consistant. Make sure it is only 1/4" all the way. When you cut your block, first measure your strip and see that it is 3 1/2 inches all the way down. Then I would suggest that you place your ruler on top of the strip, line up the bottom of the strip with a horizontal line on your ruler. See if at the 3 1/2" mark if it is a square in both directions. Then cut.

You may know that the strips cut vertically from selvage to selvage has more stretch than if you cut it horizontally. That too may help.