Measuring very low resistances in a voltage divider is very difficult unless you consume large currents. A better method is to feed current from a pin through the resistor and then a capacitor to ground. You can monitor the voltage across the capacitor to see how fast it charges and from that calculate the resistance.
Alternatively, you can use a fixed current source and amplify the small voltage dropped across the resistor. If you do that, I suggest you use a four-wire measurement to get better accuracy. To do this, feed the current through two wires to the resistor and measure the voltage across the other two wires, it eliminates the voltage dropped in the wiring which might be quite high compared to the value you try to measure.
Brian.