I use to be in position 2 for one keyword, but i dropped to 4. Well, I recently changed my link structure, so do you think that is why?
Nothing to worry about... Google serp changes every day even several times a day. Also this may depend on different DC.
also, might not be anything you've done 'wrong' - could be that the sites above you have done something 'right' - more backlinks, tidied up their site etc.
On my blog and some of my pages that get tons of traffic i have used Qassia - no aff link. You get points for your content and your articles and backlinks generate really good traffic.