OK, now first the disclaimer. The following is a SWAG and only a SWAG, cos I've never had one to tinker with.
I have an Alden ZZ with built in cams that requires you to move a lever when you do stretch stitches so the machine can do it's back and forth dance as it sews. If you leave this lever in the wrong place on certain stitches, it will sew backwards.
Perhaps your machine has such a lever or knob or switch and it's in the wrong position.
Something to check at any rate.
I agree about taking the top off and cleaning and oiling it good.
Also, by all means find a real sewing machine repair shop, not just a store.
Joe