He's jealous of your cousin without knocking it..

Basically, you had made plans to meet up with family, but completely forgot, and as you were cuddling with your boyfriend, you got a text from your cousin asking, "You still coming??" So you had rushed out of bed, leaving Megumi in bed

You and your family went to get food, and you and your cousin took a picture of the two of you at the place holding up and showing your food, his arm around your shoulder, and posted it

And of course Megumi saw this photo without knocking it was your cousin,

So first, you just dich cuddling now posting with some random guy? 5 minutes after posting you on a text, it was your boyfriend..