FIrst the iron, try this, no constipation. It got my iron up quickly and I felt a lot better. I didn't realize the crankies and exhaustion were for a reason.
http://www.vitacost.com/Enzymatic-Th...nergizing-Iron

Carb loading is generally done a few days ahead, have more carbs than usual so it's not such a shock to your system. Then you fast overnight and drink only water before the test is over. Not tons of water, but I was so thirsty.