Only God knows your granddaughter's heart, and whether her commitment to Christ was real or simply a passing emotion. But either way, continue to pray for her, and ask the Holy Spirit to convict her of her sins and bring her to repentance and faith in Christ.
Never forget, Satan's chief goal is to keep us from God and if he can't do that, then he will do everything he can to discourage us or make us turn our backs on Christ once we have come to Him. Perhaps this is what's happened to your granddaughter. Jesus spoke about those who seem to welcome the Word of God, but then "the worries of this life, the deceitfulness of wealth and the desires for other things come in and choke the word, making it unfruitful" (Mark 4:19). This happened in Jesus' time, and it still happens today.
And Satan can be very clever! He loves to sow the seeds of doubt in the hearts of new believers, and he also tries to convince them that his way is better (and more fun) than God's way. But it isn't true as your granddaughter will discover.
Let her know that you love her, in spite of her rebellion. And ask God to help you not only to warn her of the dangers she faces, but to assure her that God still loves her and wants to welcome her into His family forever.