I agree that Vitamin D deficiency could be part of the problem. I seldom go out and recently realised that I was becoming more and more depressed and weak. I couldn't concentrate for long and couldn't be bothered to do things I'd enjoyed, before.
I did some research and concluded that, since I am indoors 98% of the time, I was getting virtually no natural sunlight. Since our bodies need exposure to sunlight in order to manufacture Vitamin D, I had become severely deficient.
I bought high-strenth gel supplements on Amazon, and a fantastic wee handbag-sized spray (target - the tongue!) that contains 1000 measured doses. Over the last few weeks I've noticed a gradual but definite improvement. I would imagine that restoring my Vitamin D levels to 'normal' might take many months, but you start to feel the benefit in no time at all.
Of course after WLS weakness and fatigue could be due to malabsorption, which seems to affect most people post-op. You may lack iron, and perhaps a whole host of other vitamins and minerals. You may even lack protein.
Your symptoms definitely need to be investigated so speak to your team and nicely insist that you be seen at the earliest opportunity. Good luck to you both xx