Why is my zigzag stitch coming out on the bottom?

On a piece of clothing you are wearing or when you sew?

When I sew

What make of machine? Is the thread threaded correctly?

Sounds like the bobbin thread is not properly threaded and/or has too loose a tension on it.
Try using a bobbin thread of different color from the top, on a piece of cloth, so you can see the threads better.

sounds like the bobbin thread is not properly threaded and/or has too loose a tension on it.
try using a bobbin thread of different color from the top, on a piece of cloth, so you can see the threads better.
I'm impressed -- a Renaissance man!

It is threaded correctly because the strait stitch is OK and it is a brother machine.

Ok I'll try that