Arduino Mega 2560 Heating when powered by battery but not ampmeter

The voltage regulator on my arduino mega 2560 is heating up (hot to the touch) when connected directly with battery (with load on the other pins of the board). When I connected an ampmeter in series to the positive terminal to the Vin pin to see how much current it is drawing, the regulator no longer heat up as much (just warm). Can someone shed some light on why this is happening? If i keep using the battery like this will it be a problem that may come up in the future?