Neko de Gomen!

From Taruby:

Shiraishi Yayoi was a relatively unremarkable school girl, although a little clumsy, before a experimental accident changed her life forever. She was forced against her will by her father, self-proclaimed genius physicist, Shiraishi Sensei into a teleporter with a cat , and like in the movie "The Fly", she combined with the cat.

Now, whenever she gets excited about anything, a pair of cat ears and a tail pop into existence. In the "catgirl" form, she has super powers, but the major drawback is that she also behaves like a cat when presented with the proper stimuli, like the sight of mice. She also has a boyfriend, but her "problem" tends to interfere in their dates since her excitement inevitably leads to her transformation into a catgirl.