Sounds like the feed dogs aren't doing their job. Simple problem would be you are pulling on the fabric or it's getting hung up somewhere. Clean it well. Make sure you take the plate off and clean thoroughly around the feed dogs, and clean the feed dogs themselves. Make sure that the setting or lever used to set the stitch length didn't get bumped or set wrong. Also, check the presser foot pressure.
And finally, make sure the feed dogs aren't lowered from a FMQ session!