Also recall how much stress Usagi is under.

In the manga, her three friends have been kidnapped, and until recently, she suspected it was maybe Chibiusa’s fault and now knows that though it isn’t directly her fault, it is because of Chibiusa they’ve gotten kidnapped. Basically, Usagi is transferring a lot of distress about her friends over to the issue with Mamoru, and also using it as a distraction so she doesn’t have to think about her other problems. If all else was right in the world, she wouldn’t be nearly as freaked out about Mamoru and Chibiusa.

In the anime, Mamoru does his whole nonsensical Twilight breakup thing immediately after Chibiusa arrives. Usagi has no clues to why he broke up with her, as the only thing that’s changed between them is Chibiusa. Therefore, logically, she HAS to assume Chibiusa might be the reason for the breakup and that just puts way more pressure on the whole situation and backs up a theory that would otherwise have been unthinkable.

and wow it’s starting to get hot and heavy on this page. Mamoru’s pretty much proving he’s still all over Usagi here.

As he says, he’s pretty worried about her because she’s so stressed over so many different things, and she’s being pulled so many directions. The fact she doesn’t feel she has Mamoru’s full support just makes things worse.