@SaitoZX
In Chapter 62, Miku said to Fuutarou to put an end to the student-tutor aka partner relationship, while she was disguised as Itsuki.
Fuutarou tried to find who was Fake Itsuki, and why did she wanted him to quit. The worry of Fake Itsuki = The worry of one of the quints, he thinks.
Additionally, Itsuki said to him in Chapter 63 that all of her sisters are acting strange for a while, something is troubling them, and she wants him to find out why, what are their worries, because Fuutarou is more suitable to the job.
In Chapter 64, Fuutarou starts thinking about what's bothering Miku as a completely seperate issue to Fake Itsuki's case, and seemingly comes to a realization...
When Fuutarou finally thought he solved the mystery in this chapter, he said it made sense for Fake Itsuki to be Ichika, since he had no idea what was Ichika's worry. But we found out Fake Itsuki was Miku. So there's the worry of Fake Itsuki, and something Fuutarou thinks he figured out as the worry of Miku. Miku asks him "If I weren't the Fake Itsuki, then what was bothering me?", because she is curious about the thing Fuutarou realized.