In addition, you are moving less - you are probably in bed. Since movement helps alleviate depression, your being in bed contributes to depression.

I think that there may be an association between the bed (the object - the piece of furniture itself) and depression. If you usually spend hours in bed when depressed, then the bed is linked with depression in your mind. Now, you are in bed for another reason - the flu - but the association still "clicks". Maybe a change of bedding - bright or simply fresh linens - can dis-associate the bed and depression for you.

These are just conjectures, of course, but I do know that the hypersomnic type of depression did make the bed and depression associated in my mind.

Also, for many of us, it is in bed that we think our darkest thoughts.