There's two great online resources can give you a quick reality check and a Big Picture sense of external algo factors that may be affecting EVERYONE - and not jut you and your own site(s).
Algoroo offers a an easy to grok dashboard showing the degree of volatility in Google SERPS over the past month, with more volatile days shown in red, the more uneventful ones in green in the bar graph. The 'speedometer' widgets reflect both a visual and numeric readout of Overall, Monthly and Yesterdays changes.
Likewise, MozCast offers a comparable but more simplistic chart of Google SERP volatility. They use a bar graph and a 'Weather' metaphor and temperature readout for how 'Hot and Stormy' the day to changes of the Top 10 SERP positions have been recently.