Amy in Ames

This afternoon when I was cleaning my Millie she got stuck on super high speed. I pressed a button and she calmed down. This has never happened before. I have noticed lately her engine revs like I am going too fast when I am not. Any ideas what her problem might be? Is this a stitch regulator problem? Belt problem? After she calmed down I finished cleaning her and then loaded a small pillow of my own to quilt. She did fine. The stitches did seem to be a bit smaller than normal but the tension looked good otherwise. Thoughts?