Is ghusl obligatory for an unmarried girl if she feels a contraction or something strange in her private parts immediately after urinating and suspects that something has been discharged without seeing it?

Ghusl for janabah (ritual impurity) is only obligatory if one is certain that one of its causes has occurred. Ghusl for janabah is not obligatory for you due to the lack of certainty that something necessitating it has exited. Even if seminal fluid (maniy) is confirmed to have exited without pleasure, it does not necessitate ghusl unless it occurred during sleep.