She really has no right to be upset at this point. He already pushed you down and tried to kiss you, but intentionally broke his concentration.
This is all just my own assumption so don't take it as a source:
Saki is playing the long game (as she should) since aside from the age gap she IS Jun's step mother, and Saki wants him to take full initiative here so if they did the deed he'd to take full responsibility for it, she almost succeeded in the past chapter but Jun had a moment of clarity alongside the phone ring which made him back down, I think at that point Saki realised it would be risky if she pushed things so she brushed the whole thing off yet she looked clearly devastated that Jun didn't commit and most likely she foresaw that he'd feel so awkward about the whole situation which will hinder her plan, and so it was after the incident things got pretty awkward at least on Jun's side, but Saki played it cool so things might return to it's normal state so she can set up the mood for him again.
Yet her hated enemy Rinko barged in to ruin her schemes, Saki has no other choice but to show her support for their relationship so the facade of the good step mother doesn't crumble, but she clearly isn't happy about it, because of that Saki might change her strategy from the cobweb to a praying Mantis.
Again, this is all just my assumption.