Right. Avoid sugars. Sugar is actual a poison. If you think you need potassium, eat a banana. You get some fluid, potassium, fiber, and other nutrients without the cane sugar.
Also, to boost your immune system, load up on Vitamin C. It's especially beneficial when you have any illness. But you really should be taking it everyday. We just don't get enough of it in our regular diet.
And as stated above, you are probably dehydrated. Most people are daily and don't know it. Even mild dehydration will leave you weak and fatigued. I know it sounds like an old cliche, but chicken noodle soup is really one of the best foods to eat when you're sick. It's very liquid, contains numerous vitamins and minerals, and its warmth is soothing to a sore throat and sick belly.
A couple of other ideas for you (or for anyone recovering from an illness): As soon as you wake up, shower and get dressed. It makes a world of difference in how you feel. If you get up and then immediately lay around on the couch in your PJs, you'll continue to feel bad. If you shower and get ready for the day, you'll feel so much better all day long.
And the sooner you get back to some normal activity, the quicker your body will snap back. I'm not saying go out and jog the 3 miles you usually jog. But instead of lying around feeling bad, take your mind off how bad you feel by making your bed, doing some housework, taking the dog for a short walk, sitting on the porch in the fresh air, or some sort of small activity. It will wake up your body and give it the go-ahead to snap out of it.
These are things that work for me. I'm hyperactive and I absolutely hate wasting time lying around being sick. When I'm really sick, I allow myself the necessary time to sleep and recover. But as soon as I'm able, I get back to my regular routine. The more active I am, the better I feel.