You referred to a batteryoperated device in your first post. Batteries are typically directcurrent (dc) voltage and current. If current is ideal dc then it is a fixed value that never varies  in that case, all instantaneous current readings are identical and rms current equals instantaneous current. In a realworld battery, voltage and current will droop as the battery discharges; in that case, current is aperiodic and entire concept of rms becomes uncertain. Also in realworld, current load of a digital circuit varies at the nanosecond time scale  probably a meaningful rms there, but you won't see it if you sample at 1 msec.
I suggest you find out what the current waveform looks like. Ideally, your sampling regime should be influenced by the periodicity of the signal.
Computing rms from samples spread over 20 msec suggests that you expect the current waveform to have a frequency of 50 Hz.
