Welcome from Pine Grove, CA!
I had the same problem after taking a Free Motion Machine Quilting class that lasted all day.
When I got home and tried to sew, I only got birds' nests on the bottom side of fabric. (and lots of 'em)
After doing everything to the tension, changing the needle, rethreading and finding that nothing helped, my DH helped me clean out the bobbin area. It was packed with short pieces of red thread that I had used at my class. It was "cheap" thread (coats and clark). We were advised at the class to only use the good stuff and it has made a believer out of me. I now only use King Tut and cringe every time I buy a spool! :mrgreen: