It is the natural fear of rejection - you possibly feel similarly uneasy about asking a girl to go out with you for the first time.
In both cases you feel uneasy because you really want the answer to be yes, but you know that the answer is just as likely to be no (possibly more so).
But, if you don't ask - you don't get.
-- Best wishes - Majikthise.
It could be that you're just nervous; we all get nervous at some point that we won't know what to say or that the call will end badly. It could also be that you're scared you won't get the job. Whatever the case is, definitely try. It shows great effort to an employer.
If it is a first time for you then it is ok. Things for the fisrt time is ALWAYS uneasy. :)