I need to wear a bikini on the fourth of july?

Okay so I'm 15 and I got mono a few months ago but after I got over it I got chronic fatigue syndrome (cfs). Basically I can't do anything if I want to feel okay tomorrow. Yesterday I walked about a quarter mile and I felt like **** today. I'm pretty skinny as it is but I really want to tone just a tiny bit so I can look okay in a bikini. I just can't do it without making myself really sick by exercising. I don't want to starve myself either. What can I do?