Were you using the base map or the Navionics card? If the unit was tracking your speed correct and showed you moving on the screen it is a map issue. You will need to go to a known point on the map and use the map offset menu to calibrate.
Simply put go the the known point say a bridge piling or boat ramp. Set to advanced mode. Go to Chart menu. Select map offset. Use 4 way cursor to move the cursor on the map to your known point on the map. Press select. The map is now offset and you should be good to go.
Remember map offsets apply to all maps so when you go to another lake use the same steps to clear the map offset.
That all clear?