Many webmasters & marketers are concerned because of the new bug in Google Webmaster Tools. The bug is causing big declines in the amount of indexed pages that are shown in the Google Sitemaps report.
There are many cases of hundreds of pages being shown, but no longer being indexed by Google. Google has confirmed that it is indeed a bug in Webmaster Tools & that their team is continuing to work on the issue.
The bug in Webmasters is just a reporting glitch & there is no need to worry if you see a decrease in your indexed pages. Google has reported that it will not affect your website traffic or rankings.