It's a mix of bitterness, overprotectiveness, and possessiveness.
Keep in mind that these girls are minors... Even Sensei being around them is dangerous in reality. One of Ami's problems is that they can all get Sensei in trouble because they are teens:
But she also thinks they'll rape, take advantage of him, and hurt him, because he's weak. Especially when it comes to girls. Makoto can literally rape him, so she's not wrong.
Ami also wants Sensei for herself, or at the very least to be involved since he's the only family she has and someone she takes care of constantly, and she's jealous like almost every other girl would be.
Most girls in this game are
possessive. Miku and Sana seem polyamorous, though, and Ami actually seems to prefer Miku to most of the girls as well:
She's not sneaky. Makes me wonder how things would be if they were all just honest with eachother, instead of going behind eachother's backs.
Ami was also likely trying to cuck Maya in particular, who she thinks is in the closet:
Ami seemed to believe Maya Prime has been betraying her since Chapter 1, and probably prior. I somewhat doubt she didn't notice things over the years, especially considering Sensei has Maya's wizard nudes on his computer, and she's apparently heard Maya having sex at the shrine pre resets.
Ami's also not exactly a fan of being betrayed by a friend or cheated on by Sensei who she considers her boyfriend, especially in her own home. Sensei is also lying as per usual and I doubt anyone would like being lied to. So, yeah, not much reason to be happy about this.
This is Ami simply being Ami.