Looks like a tension problem, even if you did not change anything. Perhaps the lint has built - up whilst you have been busily quilting. If you have furry cotton, it will do this. Give everything a thorough clean and check there is no lint in the tension discs or stuck in the bobbin race, and take off the plate and dust in there too. Then re - thread your machine and make sure the bobbin is in correctly. Hope you have better luck after that. I once had a problem of knotting underneath, and it was the actual bobbin that was not completely circular. Threw it out and had no problem with the other bobbins.