After watching today's episode, I am sure everyone is guessing what is the past secret. I have my share of imagination too. I am a food blogger, not a writer, so it could be very plainly written.
Here it goes.
1. Razia and Zoya's mother are cousins or friends. Both are from rich background. (based on Razia's boasting about how her grandfather's house used to have such big silver plates)
2. Zoya's mother and mamu were married and they had Zoya. But Razia's eyes were on Mamu (no clue what was the attraction or reason)
3. Shireen is mamu's mu boli behen and Rashid falls for her and cheats on Dilshaad and marries her lying that he is not married.
4. Razia comes to know about it and blackmails him and since he was not in a position of leaving Dilshaad for good, he listens to Razia and sets zoya and her ammi's house on fire.
5. Razia marries mamu. (how ? don't know yet)
6. Razia keeps her mouth shut but somehow Shireen comes to know about Dilshaad and leaves Rashid and Ayaan and goes back to Mamu's house.
7. Razia convinces mamu to give Rashid an ultimatum to leave Dilshaad and come and live with Shireen and them. Razia again uses the blackmailing technique to get it done. (Why? it is a good thing to keep your secrets under your control)
8.That is when Rashid leaves and goes to their house for good and becomes Razia's slave.
If I may, in IPKKND, Khushi's mother's past was interwined with Arnav's disastrous childhood. Now, if this theory is correct, Rashid is responsible (indirectly of course) for zoya's lost childhood. Will she forgive that and still love Asad? We all know the answer to that question, but that is something to brood about.