Is feeling sad and depressed a sign of detox ?

Last time I felt absolutely miserable on day 3. I thought it was the general lack of food, and the fact that what I was working on was not being successful.
Almost like a light switch it disappeared on day 4. No food and the thing I was trying to fix wasn't any better.
Is that a sign of detox ? I ended it on day 5 much to my own disappointment. But in a few weeks I will be returning to a long fast and even in the mean time I'll be fasting for 4-5 day periods and may encounter it again.

Thanks.
Srinath.