Where did the "s" go?

During the past few weeks, I have noticed that when I am saying the contractions "isn't" and "doesn't" I often accidentally leave out the "s" and instead say "i'n't" and "doe'n't" (I left the "e" in the spelling here to emphasize that what I am saying does not sound like "don't").  Observe that apostrophes appear twice in each word because I'm leaving out "s" and "n" when speaking and, thus, forming double contractions.   Of course, I immediately attributed my dropping the "s" to my return to North Carolina.  After all, the southern accent is a bit more pronounced here than in Virginia.  At first, evidence seemed to support my claim.  While walking through the mall or visiting with people, I constantly heard others sprinkling "i'n't" and "doe'n't" into conversation. 

The double contractions, however, are not limited to southern speech.  I now hear them while listening to people speak on national television shows.  So, where did the "s" go?  Why did it leave?

I'm determined to bring the "s" back to my speech.  After all, there really is no need for double contractions here.  In addition to carefully thinking about saying "yellow" instead of "yallow" (which is a southern thing), I'll be listening for the "s" when I shorten "is not" and "does not" while speaking.  Of course, my task would be easier if others also would include the "s"!