I think that better phrase exists: "Mam motyle w brzuchu." (check any search engine).

Nope. It's not Polish, it's just artificial direct translation of an English idiom.

How do you recognize which one is artificial or not?
http://www.gandalf.com.pl/o/mot...big,348998.jpg

A book title can be anything its author or translator wants. It doesn't mean that the title makes its way into a languague as a commonly used and recognized idiom. BTW this particular title doesn't seem to be very fortunate.

Googled "serce bije z podekscytowania" returns 380 results; googled "motyle w brzuchu" returns 1,180,000 results.

I don't know any other way to find out which sentence is more common. What is your way to prove that "motyle w brzuchu" is an artificial sentence?

I just speak Polish.

I speak as well - it's my first language.
We can say that in the country of 38,000,000 people 1,180,000 will use "motyle w brzuchu" and only 380 will use "serce bije z podekscytowania".

It's not question of whether one is a native speaker or not, but about the level you feel the style and what's appropriate or not in cultured speech. Literal translations of English idioms is not. It is "pig Latin", nothing more.

One more thing I have to add: "motyle w brzuchu" and "serce bije" are different feelings - it is like matching dizziness to head ache (they can occur at the same time, they are near, but they are not identical).

I don't think so.

BTW, the Polish sentence is a translation of Japanese 胸がドキドキするわ. I think the feeling concerned has a lot to do with excitement.