HELP uneven trinity stitch. piece not square

I’m knitting a throw pillow. The whole pattern is in the trinity stitch. The finished piece should be 18"x18". I have one corner that is hanging. So the piece is not square, but an odd shape. I have the correct amount of stitches from start to finish. Used med. weight yarn and sie 8 needles. What did I do wrong???

I’m not sure why your piece is askew but if the bumps from the trinity st are all ok, I’d try blocking it. It should correct a certain amount of the shape and bring it back to square.

It may just be biasing for some reason or another and may square up when you wash or block it.