6v6gt:
Maybe for testing, use a simple sketch which explicitly sets D10 to either high or low instead of sketch you have posted.
Blink is a great one for such tests. One second on, one second off. Very easy to follow with your multimeter.
If indeed you power through the body diode, you switched the drain and source. Carefully check the pinout against the data sheet (and mind that sometimes you get top views, sometimes bottom views!)