1. Spider-Girl (Anya Corazon): Anya Corazon gained her powers from a mystical spider-themed amulet, bestowed upon her by the Brotherhood of the Spider (a secret society of spider-themed heroes). The amulet contained a spider spirit that bonded with her, giving her superhuman abilities similar to Spider-Man's.
2. Spider-Girl (May "Mayday" Parker): Mayday Parker is the daughter of Spider-Man (Peter Parker). She gained her spider-powers through a genetic mutation, inheriting the same abilities as her father. This is explained as a rare occurrence, as Peter's powers were originally caused by a radioactive spider bite.
Therefore, the origin of Spidergirl's powers depends on which character you are asking about. Both characters are distinct and have different explanations for their powers.