Kare no Shiranai Himitsu wo Irete. The Animation

It had been two months since they started dating, but Yui and Naoya still weren’t used to being a couple. Just then, a man passing by called out to them. “Yui-chan?” At that moment, Yui was speechless. Standing there was the man who had been her first. Before they started dating, driven by curiosity about sex, Yui had slept with him. The man threatens to reveal her past actions to her boyfriend. And so, once again, Yui finds herself harboring a secret she can't tell her boyfriend…

(Source: PinkPineapple, translated)