Ok, so to be clear since a lot of people seem to be getting details wrong - This isn't out of no where. They've been living together for years since the end of the high-school arc. As far as her needing to talk about stuff, she does, BUT its not about what everyone seems to think it is. She was in a state of depression and struggled to talk about it. What DIDN'T happen was no one cheated on anyone, or even flirted. That was all in her own mind from her own depression. Shit like this happens in long term relationships. Notice, however, when shit hit the fan, AYA went to MITSUKI first and Mitsuki stepped up, put her own problems to the side, and did what she had to do for her partner! Lastly, I would bet good money, based on past events for her, that Mitsuki sang her problems in the wordless (to us the readers) guitar song in this chapter. That's how she deals with grief, something she struggled with this time because it was her own music that was causing the problems she was having. So NO, this wasn't out of the blue or skipped steps, this isn't an unhealthy relationship, and this is normal!