I had a machine I hadn't used in a long time and it needed to be serviced - badly. when I got it back, it sewed backwards!
Took it in, the tech got it to sew correctly. Went home - it sewed backwards. I went out and bought a new machine,
After I followed the instructions on the new model, I discovered I had put the bobbin in backwards on my original machine. I tested it, and yep! It sewed straight as can be.
Try the bobbin and see if that helps.