Personally I believe the sh!tstorm she was going through was so big it slipped her mind. That and that they were from opposing factions that were fighting each other... so... idk... maybe it all just snowballed... ╮(￣▽￣)╭
Worriless wanted to forget all the memory so they performed surgery to change her memory and replace it with ye wanwan the real ye wanwan is dead
Because that time she is not strong. All her mind become blank because of his grandpa and many things happened. She's so depress. She can't tell to Yehan that they had a son and even didn't ask what's his name because of her pride. Also because of her action especially her ring and Yehan Misunderstood it. He thinks he is nothing to her just a fling.
The true mother and father of tangtang is Ye Wanwan(Worriless Nie) and Si Yehan(Lord Asura).Then there was a chaos that make wanwan want to forget her memories,she is inlove with Emperor Ji but later fall in love with Si Yehan(So you can consider them couple).
In the novel it doesnt explain why wanwan forgot to say that they have a son maybe because of the grampa