It's just a guess and when you see the chapter maybe you'll realize, mori sensei said that if they kept in communication every day either by mail or Line, soyo , kano and tsugu answered yes, while momoe's answer was something like ''not so much''. I don't know if that made momoe think that the other girls were straying from her, you know those things they say about momoe that sometimes she is scary or sometimes she is very strict. Momoe may not seem to care about those things, but sometimes you can tell that she cares a little bit.
The twitter message later says that they were on a video call, I guess on that video call Momoe realized that he had been thinking about it and was completely wrong and for that reason she put the message '' I was overthinking as usual''