Ut oh.....that doesn't sound good. Maybe its just a tension thing, but if you didn't mess with the tension, then that couldn't be it, right?
I just put on little drop of oil on the "wick thing" at my bobbin case and that's mainly all I do, except for the daily cleaning of lint. Anything else with my machine, I take it in to be done.
I sure hope someone can help you out!!!