That's strange that it's only on the LEFT side of your body. It could be a sympathetic nervous reaction to something happening elsewhere in your body.
It sounds like a lack of magnesium. You could try a magnesium supplement, although I'm not sure if you want to stray from the Master-Cleanse protocol.
Alternatively, taking a detox bath with Epsom Salts (magnesium sulphate) and baking soda is a good way to detox plus your skin will absorb some of the magnesium, and since it doesn't activate the digestive system, it's ok on the strict MC. Click here and check "detox bath" for more info: