The second one should be "a little nervous", but other than that they're both fine. "Somewhat" has a formal tone about it, so if you want to sound casual you should use "a little", "sort of" or "kind of" like Rek suggested.

The only difference is that they use different words, but the meaning is the same. I adore the "some-" words: "somewhat", "somehow", "somewhere", "someday", "someway",... To me they are poetry. That's why I prefer #1.

I was kind of nervous... , seems more natural to me.