The CV's never showed how Nilesh and Anita met or how long they have known each other. I have a feeling she knew Nilesh's business connection to the Modis. Subconsciously, she saw Ahemji in him or hoped she may get Ahemji back through Nilesh. She prepared to marry him still seeing her ex-boyfriend in him. When she saw Ahemji in person on her mehndi, it was a jolt to her. She realized that Nilesh wasn't Ahemji. That's why she cried looking at his old pictures. She was going through the motions of marriage because she probably couldn't back out.
When she saw Ahemji at her wedding, she probably was in a dilemma. If Nilesh and his father hadn't called off the wedding, would she have still gone through with it? If yes, would she have hoped to start an affair with Ahemji since both families knew each other? She did faint, but was it the calling off of the wedding that made her faint or did her obsession with Ahemji come back to put her in shock at what she was about to do. She realized that she was going to give up on the love of her life forever and that made her faint.
Once she became conscious, her defense mechanism kicked in. She saw Ahemji surrounded by his wife and family and subconsciously, to protect herself from turning a psycho over Ahemji, her mind made her think that she was married. She tried to play as a newly wed with her pretend husband and subconsciously, liked being in Ahemji's and his family's care. That's the reason she never questioned anything that a normal person would.
Now that she is out of her shock, her obsession over Ahemji has started full blown. In order to justify her abnormal behavior she is desperately trying to find pointless clues that prove to her that Ahemji still loves her and is unhappy in his marriage. She is going to become more and more erratic in the coming days.