I am aware of the Panda update but I don't think the changes occured in this period?
Your symptoms do sound a lot like some Panda reports we've seen. Panda updates have occurred at more than one period. We're trying to keep track of all the dates, and the list is here: Panda Iteeration Dates [webmasterworld.com]
At this point in time, there are 8 Panda dates that we are certain of:
Feb 24, 2011 April 11, 2011 May 10, 2011 June 16, 2011 July 23, 2011 August 12, 2011 September 27, 2011 October 13, 2011
The last two dates (and Oct 13 in particular) were followed by several smaller "aftershocks" on various specific dates - Matt Cutts calls that "Panda Flux".