I had this kind of "thing" recently and after lots of searching took a look at how thread was feeding off the spool. It had slipped out of one of the tension guides so basically no upper tension and a big mess on the back. Just another thing to check out. FYI, this happened while I was quilting. All was well, then, suddenly a mess - not sure why it popped out but as soon as I rethreaded and made certain all was as it should be the problem went away.