Sounds most likely a moon shadow Kerrin !
Here is a 'freeware' program that when you set your time, location etc, this will show what is currently happening with Jupiter, its Red Spot, and all four Galilean moons with transits and shadow transits.
You can go Forward or Back in time - minutes, hours, days, weeks, years and it will show exactly what going on at that time - very good for planning visual & imaging sessions
http://www.astrosurf.com/rondi/jupiter/#