Look at the supplied curves, guys. According to the graph, at -5V Source to Gate, and -5v Drain to Source, Current should be ~5A. Using R = Vdrain) / I(drain), the FET has 1 ohm resistance. All that can be expected across the 0.1 ohm resistor is 9% of 5V.
It should get hot quickly at ~25 watts. I bet the voltage dropping with time across the resistor is due to the out of range heat building up.
I bet you could brand your thumb with the imprint of a MOSFET if you squeezed it to check if it's hot.