I was checking Harriet Hargrave's book, Heirloom Machine Quilting (4th edition), to remind myself whether knots on the bottom meant I needed to tighten or loosen my thread tension. It turns out that the little thread dots on the bottom mean either that the top tension is too loose or that the bobbin tension is too tight. So how are you supposed to know which to adjust?
Fortunately, on page 37, she includes a sewing machine mechanic's tip for testing tension when sewing regular seams. If you don't have her book, I found similar information in this online article about adjusting sewing machine tension. So I am happily sewing again.