Momo decided to share her past with Shamiko, and everyone else because Momo is dumb like that. Shamiko thought it would be a special moment between them but Momo just tells everyone.
Also we get the backstory on their fancy date chapter from Momo's perspective this volume. Momo gets advice from everyone for how to prepare for the date. We need more chapters from Momo's perspective since she's way dumber when Shamiko isn't around. She's always playing it cool and aloof around Shami.