Thank you so much, everyone! I hadn't even thought of ripping out the seam to see where I went wrong. I'll do that, as a learning experience. I think it happened because I slipped with the rotary cutter when I got to the top of that piece, so it was a little smaller. Then I forgot to adjust for that when sewing the seam, so it ended up too small.
I really, REALLY had a lot of fun today!