You would need to look at the parish records (birth/baptism) for Alstahaug, Nordland. Those records are online and available for careful reading. At least her father's name should be recorded in that time period.http://www.arkivverket.no/URN:kb_read?idx_fylke=18&idx_k...
I suggest that you search the time period ABOUT 1819 since I don't know the source of that date you've given. An English option is available by clicking on that word from either the left hand column or the blue link bar along the bottom of the homepage but of course the original records will still be in Norwegian.
Be sure and read the instructions that are available from the Digitalarkivet for navigating the scanned records. The instructions are available in Bokmål (official Norwegian), Nyorsk (Norwegian), Davvisámegiella (Saami), and English.
Recommended basic reading are the "Startsiden" [Norwegian version]/"Main page" [English version], "Brukerveiledning" [Norwegian version]/"User's guide" [English version] and "Om tjenesten" [Norwegian version]/"About this service" [English version].
Good information about translating the formats of Norwegian parish church records during various time periods, many of the basic terms used and understanding how to use the information should be studied at this web site -http://homepages.rootsweb.com/~norway/na20.html