What came to my mind is...Are you sure your needle thread is still correctly in place in the tension disks and in the take up lever.
When you thread your needle be sure the presser foot is up so the tension disks are open. Also be sure the needle thread is in the take up lever. If it pops out of the take up lever you will get a birds nest on the underside of your fabric.
And lastly, be sure the presser foot is down before you start FMQ'ing.