When does metabolic rate slows down?

Hi, I've been exercising for some time now and my calorie intake is around 3600 calories but this is the first time I got seriously sick and it's been 3 days now since I've been just lying in bed doing nothing...
My question is when will my metabolic rate slow down? In other words, when will my body require fewer calories to maintain my weight (since I'm sick, it's quite hard for me to push my body to keep eating 3k plus calories)?

Thx.