Lucina's letters

This is sort of an AU (alternate universe) of awakening. The war has finished. everyone knows who Lucina is. Lucina had fought alongside Robin and Chrom. Then suddenly Lucina disappears. The only thing left is a note that said "I'm sorry. You will hear from me. ~Lucina." Months after Lucina disappeared Robin gets an letter. It's from Lucina. Why did she leave? And why is she sending a letter instead of visiting?