Jennifer lies about being a virgin to a Satanic band and thus is possessed by a demon after a Satanic ritual. The only way she can survive is to eat humans.
Her friend Needy sees her kill her boyfriend and after a fight where Needy rips off Jennifers BFF necklace, she stabs Jennifer with a box cutter in the heart killing her and releasing her soul.
Since some of Jennifer's blood got onto Needy, she gets some of Jennifer's powers. Needy is locked up in an insane aslylum but escapes to kill everyone in the band who turned Jennifer into a monster.
Thanks Steve B!