I am freemotion quilting a quilt and suddenly my machine started skipping stitches. The only thing that I did was change freemotion feet. I went from the plastic one to a metal one. What could be causing this? I have never had this problem before.

The only time I ever had a problem with skipped stitches was when I had accidentally damaged the needle. I replaced the damaged needle with a new one and the stitches were just fine. Maybe you nicked the needle a bit when you were changing the presser foot?
-Christine

yes, that almost always indicates the need for a new needle.