It's not just you, many webmasters are reporting seeing index counts fall in the Sitemaps report within the Google Search Console. Google is working on a fix.
The Google Search Console, aka Webmaster Tools, has a new bug that is concerning many webmasters and marketers. Many webmasters are noticing huge declines in the number of indexed pages being displayed in the Google Sitemaps report.
I’ve seen cases of hundreds of thousands of pages being displayed as no longer being indexed by Google. Here is a picture of one such report showing a huge drop in index counts on July 14, 2015:
There are dozens of examples, as I explained above.
Google is indeed aware of the issue and said they are working on resolving it. Gary Illyes from Google told me on Twitter “it’s confirmed” as a bug and the Google team is “working on it.”
So no need to panic if you’ve seen a decline in your index count in this report on July 14. It is just a reporting glitch and doesn’t affect your traffic or rankings in Google.
Original Article: http://searchengineland.com/google-sitemaps-report-bug-may-show-huge-decline-in-indexed-pages-225468