No, your Keldeo is not hacked/glitched, it's because Keldeo cannot change form in Black/White because Resolution form wasn't revealed back then. Change it in Black/White 2 in the area behind Alder's house. That's also why you can't change Kyurem, Landorus, Thundurus, and Tornadus' formes either. Those new formes were revealed a while after the sequels were first revealed in Pokemon Smash earlier this year.