I would suggest going to see your doctor again or finding a new and better one. We can't diagnose what's wrong with you, but it could very well be a result of the extreme calorie deficit you were running. You BMR is 1500 calories give or take a few, and you were doing an hour spin class, plus weights. Your body went into starvation mode.
If you've been diagnosed with mono you should not be doing anything beyond sleeping as much as possible and other essential activities (class, work, ect.). It sucks since you want to be active, but its the price you pay for sharing that drink at a party or kissing someone who has it. Plus you don't want to risk rupturing your spleen or your life will suck that much more.
Just rest, go see the doc about the water retention and be patient.