Yes it's happened to me too in the past. Very frustrating. Google has just had an update so that could be an influencing factor. You may be left with no other option but to "wait and see." My guess is that your se results listings will creep back to their previous levels over a period of time.

If you have been adding pages containing decent content Google has this infuriating habit of penalising you first by de-indexing multiple pages before acknowledging that your site is actually improving, and indexing your new pages. Regrettably it's more of an incentive to cease your webmaster activities than it is to forge ahead with enhancements.