Be strong, Igarashi. Don't let her bamboozle you this time. You may not want to believe what you heard, but don't try to rationalize it into meaning something else. Yes, Chiyo and Ookuma have been deceiving you all along. Not only did they know each other back in Middle School, they were also more than mere acquaintances.
Even now, Chiyo is perfectly comfortable talking to Ookuma just like in the past. Not only does she not hate and despise him and not want to come into contact with him, like she claimed, she was obviously affected by his confession. She even covered him with a blanket (you didn't notice?), not something she would do if she really did not like him.
Igarashi, perhaps the sensible and most logical thing for you to do is to cut ties with both Chiyo and Ookuma. Since Chiyo does not value and respect you enough to be open and honest with you, unlike how she is with Ookuma, it's likely that lies and deception will continue to be a constant and recurring facet of your relationship with her ad nauseam.
But don't despair. Unlike Chiyo, there is another woman who actually likes and admires you for who you are.