Good job so far hun, you should be very pleased with yourself. As for the tiredness, it does come with territory! For one you have just had major surgery, which your body is using most of its energy to heal. Secondly you have dramatically reduced your caloric intake so your body is conserving as much energy as possible. And lastly, you have become a fat burning machine so your energy is being used for that purpose as well.
Take naps as needed, try to get as much protein in as you can and baby your body as much as possible. Once you're eating solid food again, your energy levels should rise and once you have healed more fully you should feel an increase in energy.
Just take it slow for now and you'll soon be back up to normal speed. Just brace yourself for added energy in the near future, it comes as a lovely surprise when your body realises that it needs less energy to run your much smaller body!
Best of luck,
Nic