I know she does not sing too often and I know she appears in the "The Millers" at the same time.

There's no doubt about her being absent from the 100th episode of this show for some reasons:

1.) She was in this show since the pilot.

2.) She sang some songs along the way.

3.) Her character was officially married to will recently, but promised not to divorce this time.

4.) Her character is about to give birth immediately.

Jayma will not sing the 100th nor appear in the pic, but she should guest appear in the 100th episode, no doubt.

http://www.eonline.com/news/525121/jayma-mays-is-probably-done-with-glee-find-out-why-on-chelsea-lately

"It is probably (the end) for me, for my character, but I'm not sure about the show itself,” she tells Chelsea Handler.
Read more at http://www.hitfix.com/tv-tattle#m1eI1erPGirlVSLt.99

http://www.eonline.com/news/525121/jayma-mays-is-probably-done-with-glee-find-out-why-on-chelsea-lately

"It is probably (the end) for me, for my character, but I'm not sure about the show itself,” she tells Chelsea Handler.
Read more at http://www.hitfix.com/tv-tattle#m1eI1erPGirlVSLt.99

I have questions about her:

Is "obsessive compulsory disorder" is the strong cause for 1.) to destroy publicity and 2.) her to leave Glee that soon?

I was wondering why the Glee producers, writers, critics and viewers abhor that Emma character and her obsessive compulsory disorder reaction too much!