Misaki-kun no Bukiyou na Dekiai

The OL (office lady) Kyoka has a handsome boyfriend named Misaki who is a university student seven years younger than her. They have been living together for three months now, but there is something wrong with him. He's staying out late, looking at his phone all the time, and they haven't had sex in a while. Has he even said "I love you" to her? Could he be cheating on her? As Kyoka's suspicions grow, an unexpected event occurs on her birthday...? (Source: Cocohana, translated)