If it's happening, I can think of a few possible explanations:
1) You were tired.
2) She was not doing a good job of making you comfortable; for example, she may have been very untalkative, which put the entire strain of making conversation on you.
3) You had oneitis for her.
4) You tried too hard beforehand to think up something interesting to say.
If you're doing a good job of living in the moment, not thinking ahead for clever thinks to say, not trying to impress her, and not yearning desperately for some smooth way to #-close, then she probably just wasn't doing a good job of making you comfortable. Some girls just make guys nervous, and it's usually because they have poor conversation skills. The upside is that every other guy will have the same problem with her, so if you really want her for whatever reason, you will not be at a disadvantage.