Arduino accuracy is 0.0048V/bit with a 0 to 5V input into an Analog pin (not digital).
So 100/20 = 5V = a reading of 1023
Thus 1 bit = 0.0048 * 20 = 0.098V.
However, if your arduino is a handheld device, that means your ground is not really ground - and if you connect across the shunt with arduin ground on 1 side side and the Analog input on the other, you should be seeing much less voltage.
For example, this 10mOhm shunt available at dipmicro.com. You would have to put 500A thru it to see a 5V across it (5V/0.01ohm = 500).
How many amps are you expecting?