Uh. Why are they having an argument now? What is going on?
Aya has been feeling insecure since chapter 98. She was clearly upset but wouldn't admit to it in chapter 100 when Mitsuki asked her what was wrong, even snapping at Mitsuki when she told her, "It's fine!" in an attempt to shut down her questioning. It was a big enough deal that Mitsuki noticed it and felt anxious about Aya treating her differently. She can tell that Aya is upset and acting weird, but she doesn't understand why.
Honestly, I think that's pretty realistic on Mitsuki's end. She doesn't know that she's a character in a shoujo manga (an important point that I think people tend to forget when they're evaluating how characters act) and she still has low confidence and self-esteem. All these interactions with other students now that she's popular mean very little to her, Aya is the only person she's let close to her heart. In Mitsuki's perspective, what reason could Aya possibly have to be jealous?
It makes perfect sense how this whole situation could lead into them both being frustrated with each other for things being so awkward and tense, but neither of them has stepped up and really tried to talk it out (yet).
It wouldn't hurt some of y'all to go back and reread recent chapters before a new one comes out, just saying. Several weeks can be enough time to forget some pretty important details.