It's never fun when you're not feeling well and often the last thing you want to do is work of any sort, even if it IS usually very enjoyable work. I've always been told that you have to go do things like that anyway, even when you're sick because that's what adults do. Now despite my own personal feelings in regards to how we treat sick people (for instance I really don't think food people should be allowed in to work while they're sick and I've read an article recently that echoed that thought. Yet so many people do it anyway!).
Anyway! I thought it an interesting experiment to write a little while I wasn't feeling good. It is a peculiar thing, particularly with a laptop because you can stretch out on the bed even though you're feeling awful and pull up a document to type in. That doesn't seem so bad right? Now I wouldn't suggest doing it with something you're seriously working on unless you've got pretty good mental clarity despite the ill feeling, but it's an interesting idea to write and really experience your sickness. Who knows, you might realize that it's really handy later when you're feeling better but one of your characters is sick.
I've spoken before about writing life and drawing from it, so here's another opportunity. The same thing can be done when you're feeling down or when you're doubting yourself. Writing the physiological and psychological effects of such things while you're feeling them can give you a really good resource to draw from later when something similar happens to your characters. Try and take everything as an opportunity and write a really in depth journal of how you're feeling, especially when you're feeling awful. The more detailed the better. After all, what better way to help the reader really feel like they're there than to write about it from an extremely knowledgeable place?