I have been pondering on this question ever since the third season, and it still hasn't been answered. so if anybody does know can they please tell me!

Oh, is that what it was? I didn't know that either!!
!chik!
P.S. Whatever you do, do not respond to this post cause it is my 666th post. If you respond, who knows what'll happen to you!!!

As Bubba said, Bitty Schamm left the show because of a contract dispute, but in case you meant that you have been watching the show trying to figure out where the character Sharona went (which is what it sounded to me like that was what you were asking), in Mr. Monk and the Red Herring, the first few minutes after the opening credit explain this by having Adrian and Dr. Kroger discussing the fact that in spite of Adrian being in deep denial, Sharona is not coming back, cleverly avoiding a bit of very blatant exposition about the whole situation. Basically, Sharona sold her house, re-married her ex-husband, Trevor, and moved back to New Jersey, and not even Dr. Kroger can order her to come back. After some thought on the matter, Adrian finally admits that it would even be pretty pointless for Dr. Kroger to fly up to New Jersey, drug her, and bring her back by force, saying that Sharona's pretty smart and would probably just escape anyway, but thanks him just the same. If you haven't seen the episode 'Mr. Monk and the Red Herring' you really should look for it, it has some pretty funny moments and explain a good bit.

I love this episode as an introduction to Natalie and Julie. The whole trash-can-on-fire and measuring out eight feet to use the extinguisher (it doesn't say approximate) is a fun bit.

OHHHHHHHHHHH!! Now I finally know. Thank you! Gotta see that episode still... haven't seen all of the third and fourth seasons yet.