I think maybe all this is to do with the Holi night when Vivaan slapped Chagan when he stood up for Imli in front of him. Chagan maybe decided to take revenge against Vivaan for this by using Imli or maybe he still has a obsession with her but does not show it and remember guys that night Imli stayed in the basti NOT haveli also thats what the Vivaan and Chagan fight was all about. He could have drugged her with something and taken advantage of her that night which can be shown in a flashback scene.
Vivaan could also be the father but since Paras has mentioned Chagan I am getting all these new ideas in my mind.