Computer Use and Depression

I've noticed that there are times after using the computer that I feel sad or easily irritated when I've been happy early on. Lazying around after catching up on sleep and playing some computer games, I decided to search if prolonged computer use can cause depression and here's what I found:

http://www.ehow.com/facts_6081962_computers-depression.html

http://www.healthkicker.com/732095208/internet-usage-may-cause-depression/

Disclaimer: I'm not depressed when doing this lol