My older daughter did the same thing. I was so excited when she went down for a nap and I had a moment to regroup. But I dreaded when she woke up because she would have a horrible crying jag that seemed to go on forever. She outgrew it - that and naps actually.
I wonder if as toddlers, they just wake up in the wrong phase of sleep. I would just hold and soothe my daughter and after a few minutes try to distract her with TV, or a snack or just about anything to stop that crying! LOL I truly miss those days.