It sounds like you experienced spoofing where one or more of your GPSs changes totally changes location. Only way to verify this is the difference between GPS coordinates and IRS coordinates. One way I fo7nd to make sure I was where I thought I was was to look at the coordinates of waypoint on the airways in Jepps and then when we fly over it, check coordinates of IRS 1 2 and 3. Then check coordinates of GPSs if you have any showing. It’s at times like this that you may get GPWS Terrain Terrain etc etc.