I guess it's no mystery that with such a low food intake you are bound to run out of energy some times. My biggest problem with it is how suddenly it happens. There is no warning! Like today I have actually had a pretty good day energy wise. Gotten some stuff done around the house, took the dog for a good walk, nothing earth shattering but for me lately it isn't bad. Then just now I was cooking dinner after a longish break, I was feeling pretty good and then -WHAM!- I am so tired I can hardly stay on my feet and I just wanna cry. There is no warning. I don't feel a little tired, then more tired. It's like running into a wall! Do you guys experience anything like this?

Are you getting your vitamins in? Getting enough fluids? Can you check a blood sugar? Are you eating enough? What's your caloric intake? Are you on meds that may need adjusting? I needed to go off one of my BP meds after only 2 weeks.
I'm not feeling like this now but I was in the beginning. I didn't get energy back til going into week 5!

I am taking all the prescribed vitamins, but of course my calorie intake is low. My protein intake is laughable, but I am pretty good with fluids.

It is due to your lack of protein. It is so crucial to ensure we are getting the right vitamins and nutrients in. Not getting proper nutrients is only going to lead to problems down the road. You should not be feeling that lethargic at this point in our journey. Please talk to your surgeon about this.