Do not go to the gym! You will just make yourself worse and then you actually won't be able to go for even longer.
Let your body rest and then only when you are better, start doing some GENTLE exercise (eg swimming or walking or something) as your immune system is low.
Don't worry about gettting fat, your bmi puts you right at the bottow of the range so that is not something you have to worry about. Stopping exericising for a week or so will not automatically make you put on weight at all.
If you really have the flue you would not have the energy to get out of bed. I had flu once and it was the most nasty thing ever. I was stuck in bed for a week solid as I had no energy and I kept being sick all the time. Even turning my head was a great effort. Even after operations I didn't feel that bad.
Stop confusing the flue with somthing like the common cold which case you probably can go to gym.
I'd be more worried about losing weight if I had the flu, your body needs as much rest time as it can to fight the flu, you going to the gym is not going to help that. Just rest up for a couple of days and then start exercising again slowly, get well soon!
You cant possibly have flu if youre up and about out of bed... with flu youd be barely able to get out of bed to go to the loo, let alone go to the gym. From the sound of it you have a bad cold, in which case best off resting and youll get over it soon.
I agree, sounds like a cold - 'flu would leave you pretty much immobile and far too miserable to even think about going to the gym. If you're knackered, don't go... simple as that. A few missed gym sessions does not equal instant weight gain...