Make sure you site is working fine... One time I introduced a bug in my web app that fubared navigation for bots. Saw a slow but steady decline in indexed pages. It was a huge site, but learned a lot from it... One lesson is that it takes 24 hours to get indexed but it can take weeks for those URLs to be updated in the SERPS and it can take months for a site to organically go away. UP & Down a little is normal. Down slowly and consistently over time is a sign the googlebot is having trouble with your site.
Another issue is that if you introduced anything that makes your page load times slower then googlebot will likely crawl less of your site with each visit and that could cause a "one time" noticeable drop in indexed pages and then it will stabilize when it finds equilibrium again.
If you are creating a large app in PHP,JSP,ASP, etc... benchmark everything and stay up to date on "performance tuning" for your language of choice. Using careful development I was able to get about 250K pages indexed in 4 weeks, but you have to really measure everything and and tune away the milliseconds to get googlebot to believe that sky is the limit on crawl speed... after all... it wants to be polite and not be a denial of service attack. It knows your page load times, but do you?
Hope that is helpful.