See how the top stitches are loose? This is a clear indication that there is trouble!
Looking at the underside of the quilt shows this mess. Birdnesting. Noooo!!!!
Ok. Breathe. Tension issues has got to be the probably the biggest terror that we face as sewers. Is it the upper or lower tension? It is clear that this one is the lower thread is too tight, pulling the upper thread in too far. I spent a half an hour ripping all this out and went to do some set up research.
I found a really good video on you tube that was very helpful. It was made by a man who sets up long arm machines and teaches people how to use them. That's a reliable source!
First thing - the bobbin was set too tight. Set the loaded case in your hand and pull on the thread. It should not hang in air like this. Jamie explained that on long arm machines, the thread should pull the case up onto its side in your hand and release the thread freely. It should not lift it off the hand.
The large screw on the side of the case adjusts the tension. I turned it counter-clockwise to loosen the tension. Then did the thread pull test until it did what the video described. This adjustment gave the correction in my tension and the stitches began sewing correctly again.
Another problem that can destroy stitches is the way your machine is threaded. This is not correct. Can you see why? The thread is not going through the upper tension discs.
Fixing tension is not as much of factor when you are using tabletop machines. Most hobby machines hide the mechanisms, only showing a dial for tension adjustment, making it somewhat mysterious. Changing those settings is frightening!
It does have to be addressed when using different threads and fabrics. Getting an understanding of tension is worth the effort. That way you do not have to feel the terror when your tension goes awry.