Chris,
This is what I think you are doing...
You created and saved your map. Then you navigated away from the map tab to a different page in ArcGIS.com. Then to go back to your map, you navigated to My Content, found your map and clicked Open to view it. Is that correct?
The ArcGIS.com application isn't currently checking to see if the map you are opening is actually the same as the map you currently have open on the Map tab. An easier way to get back to your map is to simply click the Map tab at the top of the application.
The reason why you are getting this message is because any small change to make to a map, even panning and zooming the map, will flag it as "changed". Thus, when you go to open one of your other maps, the system will tell you that the map you currently have open has changed, giving you the chance to save it before you open the new map.
You can always just click "yes" to open a new map if you don't really care to save any changes to the currently open map.
Hope this helps,
Mike