Hmm, you say the only thing different is your bobbin thread. So you have used the YLI monofilament before without issues in you HQ? Is your YLI monofilament cross wound on the spool or stack wound? If it is stack wound and you are putting it on a vertical spindle it is causing the thread to really stretch when feeding through all the tension discs. Once it is sewn in the quilt all that stretch tension is off the thread and it is springing back to it's original size, as Tartan suggested. If you have used this spool without issue before, I am thinking it has gotten down far enough on the spool to cause the problem and stretching badly as it feeds through all the thread guides and tension disks on you HQ. A stacked wound spool will behave itself much better if it is fed on a horizontal spindle, so the spool can actuall spin as it unwinds, not feeding off the top of the spool. If it is cross wound your top tension may be way too tight or you have yourself a defective spool (it can happen!). I agree with the others, switch out to a light pink or white thread and see if that doesn't help matters. I am sorry you are going through all this. It is very frustrating but think of it as a learning experience.