Random Riddle: Just Add An S

A word there is of plural number.
For to ease and tranquil slumber.
Any other word you take,
and add an “s” will plural make.
But if you add an “s” to this.
So strange the metamorphosis
Plural is now plural no more.
And sweet what bitter was before.

What is the word?
 

Random Riddle: Just Add An S