P.M.
ALL behavior is a means to get our needs met. Defiance, whining, pouting, tantrums – also affection, snuggling, humor. Any particular behavior may not be a good strategy, and for little ones, it's often a bad strategy. Children throw tantrums, scream and kick – not as a deliberate, planned manipulation (at least, not those first tantrums), but as an intuitive way to try to meet their own NEEDS, one of which is to let out frustration when it becomes too much for them. Depending on the results they get, tantrums may stick around or eventually go away.
If your daughter senses that she has hooked you emotionally, she feels at least a little more powerful. Even a negative reaction from you tells her she's had an effect. Because she is feeling very little power, that's very, very attractive. She'll gratefully take any power and control in whatever form she can find it. Wetting herself is probably a way of telling you that no matter how much punishment or deprivation (in her opinion) you heap upon her, she can still control that one little area of her life.
More empathy and tenderness actually have far more positive effects on my grandson than parental anger, warnings, timeouts or other punishments. I've been using the techniques outlined in the book How to Talk So Kids Will Listen, and Listen So Kids Will Talk. You can read part of this really practical guide to connecting with your kids here: http://www.amazon.com/How-Talk-Kids-Will-Listen/dp/038081.... I hopw you'll be impressed with the possibilities.
So before deciding how to deal with her behavior, I'd try this: stand in her shoes and consider life from her angle. Notice all the frustrating, controlling, maddening and discouraging situations for a new little person with little choice and limited language. From there, you might find creative ways to reduce as many of those frustrations as possible. This in turn should reduce her negative strategies to deal with her own frustration.
I hope you will NOT to think of her feelings as "wrong." She really doesn't choose them, any more than you "choose" to feel angry when she throws yet another tantrum or pees herself again. Anger and frustration are natural, and completely legitimate. If you discourage them by shaming or punishing, the feelings won't go away, they'll just go underground and emerge in some other way.