Welcome and congratulations, Johimbe. You didn't say what procedure you had, but each type brings with it different recovery responses. But all of them result in fatigue. Your body has been through a surgical procedure, which is difficult enough. But your digestive process has also been changed, and several of your organs are affected by that. What you want to do is give healing your highest priority. Follow the post-op eating plan you doctor gives you, and if anything feels bigger than you think it should, call your team and be prepared to hit your local initial care facility for a check up.
Caffeine is a common topic in this group. Did your doctor or nutritionist tell you to give it up? It seems common nowadays, but I had my surgery many years ago, and was not cautioned against it, nor did I ever give it up. I'm a one-cup-a-day coffee drinker and there's no negotiating about that. I'm having that perfectly made, absolutely delicious, essential element in the morning. Now, I know there's caffeine in a lot of other foods, so I would definitely be willing to give any of those up, but that little bit of coffee is as important as any healthy meal I eat.
Make sure you tell your team members how you're feeling, and if caffeine seems like an issue, ask them if you can have one drink containing caffeine every day.
You're not on any kind of weight-loss diet. You had WLS for medical reasons, chief being obesity and what secondary disease comes with that. You're trying not to become diabetic or have terminal heart problems. My not-so-humble opinion is that a high percentage of dieticians are biased and will recommend you cut things out that they don't approve of. If someone had said I had to give up caffeine, I would have asked why, and made them specify how that would be bad for ME, because I'd want to know why they think they know me when they just met me.
Anyway, I never gave it up, and I followed the post-op diet that was given to me to the letter. I ate better after surgery than before, by far, and started feeling better right away, except for the weariness. I had an open procedure which was general anesthesia surgery, so my recovery time was longer than most. I allowed myself to stay in bed for more than a week and had a roommate who helped me prepare my liquid and pureed diet phases.
Just remember, you are unique. If your post-op diet is making you weak and tired, talk to a medical professional about it. You may reveal things about your unique physique that will allow a dietician to allow you an exception. In this business, the more you tell and the more honest you are, the better your odds of successful recovery.
Best of luck.