Nope - I tend to have the opposite problem...but neither one is any fun.
I assume you've talked to your doc about this...?
This is gonna sound crazy, but I work with horses, and I know when one of them is eating lots of food but still can't keep weight on, it's often a mineral imbalance...often Selenium. Obviously, horses and humans are very different, but could it be something like that? Sounds like you are eating normally....but for some reason your body is not able to absorb the food. I don't know what sorts of things causes that in humans.
I'm sorry you're dealing with this. It must make you even more fatigued, on top of the Lupus stuff. :-(