Zooms in by Itself!

I am working in my iCloud calendary, then all of a sudden the screen zooms in, so much that I can't work my calendar activity. I don't think I am doing anything to cause it to zoom in. I am not touching the track pad, only the mouse. (I probably am doing something to cause it.) Once it zooms in, I can get it to zoom back out. I have tried the finger pinching activity with no success. Suggestions on what is causing it to zoom in and how do I get it to zoom back out?