As far as I can see from your code, the your Arduino is running at 5V.
5000mV / 1024(10-bits) ≒ 4.88mV
If this specification is correct, 4.88mV means a current of 3.3A.
The ADC goes from 0/1024 to 1/1024 for the first time with a 3.3A current.
What is the current flowing during measurement?