Should I be worried about this?

While I've been sick with the flu, I've managed to lose about 5lbs. I haven't had an appetite but yesterday I started forcefeeding myself. I'd think the entire half gallon of OJ I drank in one day would have been enough calories but I still lost.

Where does illness weightloss come from? Fat stores? Muscle? Fluid? Should I be concerned? Eat more? I've been drinking lots of water and juice and I haven't been vomiting and no diarrhea.

No you shouldn't be worried.  

I will say (and I am a massive lover of juice) to stop drinking that much juice.  And if all you had yesterday was juice, then yes you need to eat more. 

Well, you are not getting a wide range of dietary nutrients. Also, when you are sick you can become dehydrated etc. You need to push fluids but you also need to try and eat some protein and some carbs (not just drinking carbs - i.e. juice).

It is natural. Plus when you are so sick, you just don't want to eat/drink. Just try to eat some things as that will also help give you strength.

Hang in there!
Traci