I would try changing both threads. What do you usually use?
If the quilting was fine and then suddenly went into pucker-mode, chances are very high that there is a thread caught somewhere in the tension assembly. Could be caught between the upper thread tension discs, or it could be in the bobbin area. Did you take a pin and clean underneath the bobbin case tension spring? Also you may need to take a strong light and magnifying glass to the bobbin area; sometimes just a wisp of thread gets caught somewhere in the mechanism.
One other possibility is that the upper tension spring could need to be changed; they wear out. However, that is probably the last thing I would do.