Return to reality

I just struck myself. I used to be so well. I'm so not well right now. This summer, I could go for so long, an hour on the elliptical or biking and now, my legs give way after 5 minutes. I mean, I dn't gain weight or anything, I'm just that sick. All my energy and strength is gone. I only have enough for mandatory tasks, any energy that I try to exert past that my body refuses to allow. this is frustrating. i don't know how to fix it.