That is too much to loose in one week, it is not healthy for the body to loose it that fast. Please see your physician as soon as possible for help.
Do you know what is exactly wrong with you, like do you have flu symptoms or anything?
I think you should call your doctor and talk over all of your symptoms. Because you have lost so much weight and now have diarrhea i make sure you are getting PLENTY of water, gatorade, something of that measure.
20 LBS IN A WEEK? Are you sure? If you are PLEASE SEEK MEDICAL ATTENTION LIKE YESTERDAY. Do something; this IS something to be concerned about.
Do you have a medical history? i.e. Diabetes
That is quite a bit to lose in such a short time, but I wouldn't be too concerned. A lot is water weight-- it is common to get dehydrated while fighting a virus, especially if you've been throwing up or having diarrhea. Losing muscle mass is also a result from being bed-ridden for a while. It is also completely natural to lose your appetite when you are sick; the body naturally wants to fast to rid itself of the toxins.
You could call your doctor to get a second opinion, but I think as long as you stay hydrated and get right back to a healthy diet asap, you should feel stronger again.
All the best,
Brittany