Measuring very small changes in a low resistance circuit
Make a decent current source that is controlled by the pin. This means that the pin is not loaded.

Remember that milliamps times milliohms gives microvolts. If your DC voltage is in microvolts like, say, 20 \$mA\$ through 4 \$m \Omega\$ - giving 80 \$ \mu V\$ - then even the expensive opamps will struggle to give good accuracy.

If you could source, say, 1 amp, then a simple cheap op-amp circuit could drive the analog input pin without too many offset errors.

The current source is only activated when needed so energy needs should be easy to manage.