AarYa consummated their relationship and Aarti confessed her love to Yash. That's why he was so understanding about it earlier with Aman, and acted like he already knew. They did it more than once. Aarti wouldn't expect that to be the reason if Yash was fine the morning after their consummation, and maybe even wanted to do it again.
So they were sexually intimate in Mumbai. Then Yash gets the phone call and feels betrayed because he accepted Aarti as his wife in the true sense and was maybe even coming to terms with his feelings for her.
If this is the case, Prashant would still be the reason. Aarti would also not have any reason to expect their sexual intimacy to be the truth behind his anger.
And then I guess... she finds out she's pregnant now, in tomorrow's episode? 😳