Exactly.
I have tried to explain in few posts as well.Fling means casual relationship-doesn't mean physical/sexual.Sheetal might have been serious but it doesn't look like Arnav was serious about the relationship.She might have had a crush on him.
According to the spoiler he tells Khushi about the fling with Sheetal on Monday.Do you think he would have told Khushi so early if he was seriously involved with Sheetal.
For Khushi even dating is a big thing-so definitely it will create suspicious/MU in her mind.
True the MU will be from khushi's side and it's more likely to be happen side the kid has same personality and talks walks and wears same as Arnav if there was no Arav khushi would have get jealous but try to brush it of thinking Lavanya was also linked up with Arnav but since Arav is there she will doubt and link two and two together jumping to the conclusion like she did after reading the diary of Arnav
Just one hope that khushi doesn't go into scarifice mode and bring Arav with Sheetal to RM would be creepy to wach he there whole time