clean the machine, rethread totally from the spool on, replace the bobbin (after cleaning out all of the lint) and try again. did you rethread or change the bobbin before this started happening? sometimes if the thread is not in the tension disks properly, just laying on them not actually in between them this can happen, that's the reason to start over and rethread, paying attention to each step, give the thread a bit of a tug to make sure it is between the tension disks. if this does not help you need to put together a practice piece and start adjusting your tension...but since the tension was good until now i bet it's either a threading issue or lint build up