OK here is some additional info. It took me a minute to remember the another version of the name. Alwinda is known more widely as Alvilda, Alwilda, Alfhild or Alvild. She first appears in Saxo Grammaticus' book "the History of the Danes". So she is likely historic, but is so far back in history that the exact circumstances may have been exaggerated through verbal storytelling. So you've kinda got a legend based on an actual person.
Here is a link to History of the Danes, with the different places that Alfhild appears:
Alfhild in Danish History
Here is a nice little summary, since Saxo Grammaticus, although interesting, is a tough read:
Alwinda