The 21-year-old singer - who shot to fame after winning UK TV talent show 'The X Factor' in 2008 - came up with the unusual moniker in honour of the persona she takes on when performing.
She said: "When I was on the show I could only show my personality to a certain extent. I didn't want people to think I was weird! But now I'm letting rip.
"The fact is I'm a loud, bubbly person, and a bit of a party girl. But it was hard to portray being a party girl on the show. Now I can be free. When I perform I call myself 'The Beast' - I'm a monster! They're going to see me go wild on stage."
Alexandra - whose new single 'Bad Boys' is released later this month - added she doesn't understand why people compare her to Leona Lewis, who won the show in 2006.
She insists they get on well and are not rivals, explaining they recently had dinner together and didn't even talk about music.
Alexandra explained to Britain's You magazine: "A lot of people have this thing where they think we're against each other because we're signed to the same label and we won the same show. But those are the only things that we have in common. Otherwise we're completely different. People will get to see that very soon"
