Have you tried resetting the bobbin tension? There is a little screw there that you turn. I put a tiny mark with a permanent marker on the side of the bobbin case to mark where the screw was originally set so I can go back to that setting. Remove the thread from the bobbin case before you set the tension.
Since the bobbin stitching was normal for just a little way and then got wonky, you may have to take it in for repair.