Google has bumped me dowm 27 positions over Night!

[SEO]

Over the weekend a site i work on was holding position 3 for specific keywords in google .com for many months two keywords say new shoes for example. Along with another 5 main keywords some 3 worded some four. Last week I noticed I got bumped down to position 8 taught it might just be some competition, but even at that, thats a good jump. So I started building more quality relevant back links maybe 10 over the last week. Now this week for my main two worded keyword which only has about 100,000 compo on it. Has been bumped down to position 30! from positin 5. No changes were made to the site I have been told. Well not on the page that has been knocked down. Has any one any rational explanation for this. Or experienced any thing similar recently. I have used the best seo practices and never use black hat tricks. So I am lost as to why this would happen. All advice greatly appreciated.:roll: