Anastasia Steele: You may call me Mrs. Grey.
Christian Grey: You insist on defying me, Mrs. Grey. What should I do about that?
Anastasia Steele: Learn to live with it.
Christian Grey: Good morning, wife.
Anastasia Steele: Good morning, husband.
Anastasia Steele: I just can't believe this is my life, that I get to live with you.
Christian Grey: I'll pick you up at five.
Anastasia Steele: I may not be done by then.
Answer: The movie's timeline is not clear, but in the book, it happened during the honeymoon, way before the next Depo shot was due, so it's not because she forgot or was negligent of it. The shot ran out too early, which is rare, but it does happen.