Be kind to yourself. Your body is still recovering from major surgery and there will be differing levels of energy. Your diet, while perfectly normal as a post op, is another primary cause of low energy as it is lacking in calories and a balanced set of macronutrients. Again, this is by design until you are further out post op. Lack of carbohydrates will cause both a decline in brain activity and physical response, especially if you are in daily ketosis. Once you are able to eat solid food the increased calories and nutrients should help you with energy. Make sure you are drinking as much water as possible (above 64 oz minimum if you can) and consuming at least 80 G protein daily. Take breaks and relax if you need some recover time.