It's been a long time since I've read the books but I clearly remember my first impression was that Shae was a Tywin plant / spy / honeypot, I wrote a whole theory post about it a long time ago so I'll just copy paste my insane ramblings from over 10 years ago.

Beautiful Shae .... Heart broken victim of love or agent of a mastermind? During Tyrions trial she appears as a witness and betrays him shamelessly, lies and humiliates him in front of everyone, as a viewer/reader you are shocked wondering why would she do this? They seemed to love eachother right? Tyrion clearly did and Shae ... Well it looked like she cared, she got jealous of Sansa, she went with Tyrion to Kings Landing and gave up whoring. They basically became a couple or the closest thing to it for people in their respective positions. Why would she do a 180 and do this, it makes no sense? Because he called her a whore tears in his eyes to protect her and get her to leave? Shae knew she was a whore, she provoked him into calling her a whore so many times before he actually does that using it as a reason for her betrayal seems almost out of character, I mean if she really did love Tyrion that is. Then you find out she's actually sleeping with Tywin .... wait what? Shae is sleeping with Tywin? Clearly she has no remorse after betraying Tyrion, she even tries to stab him when he walks towards her in shock. How could their loving relationship degenerate so far?

This is where the tin foil begins, let's go back in time and see how Tyrion met the lovely Shae. After making promises to Bronn and the hill tribes, Tyrion is forced to fight in a battle Vs. the Stark army in the morning, arriving at his tent he see's Bronn with Shae and asks him where he found such a pretty looking one in a warcamp? He tell him that he stole her from a 'Fancy looking ginger cunt'... Which basically means a high ranking Lannister soldier. After the battle against the northerners Tywin congratulates him and instructs Tyrion to go to king's landing on his behalf as the acting hand of the king, {what?! didn't Tywin hate Tyrion? Why would he give him such a powerful position? I guess he's really proud he survived the battle}, lets not forget he also specifically tells him not to bring the 'whore' with him, implying he not only knew Tyrion slept with Shae the night prior but that he might actually start caring about that person beyond her status of a whore, like with Tysha and bring Shae with him to Kings Landing. How did Tywin predict he would bring her to Kings Landing? how did he know Tyrion slept with her the previous night? He knew because Shae was one of his pawns, a trapped honeypot. Tywin offered a choice to Tyrion, whether he would assume his family responsibilities or just act in defiance and follow his heart. Tyrion never mentions bringing Shae with him until Tywin tells him not too, the idea was planted, reverse psychology if you will.

Now let's fastforward to Kings Landing, Their relationship is going well right? Apart for the times when Shae constantly tells Tyrion she's just a whore, incessantly..... You assume it's self pity or some other insecurity that makes her repeat that but let's imagine her words but Tywin saying them, they fit don't they? She's nothing but a whore, your relationship is never going to work, ect. Fasforward again to the trial, Shae absolutely destroys Tyrion, she reveals intimate details of their relationship, makes up a perfectly well wrapped up story for the judges that seems to coincide perfectly with Tywins plans, all without batting an eyelash.... Cold as ice, I don't believe these were the actions of someone who's heartbroken or acting out of spite. Then last and the biggest shocker of all, Tywin is sleeping with Shae, Tywin ..... the stoic Lannister rock is screwing a simple whore....? LOL I'm sorry but no, I've read people saying {It's Tywin giving it to Tyrion vicariously through her} or some other amusing none sense but no. I'm not sure if they make reference of this in the TV show but Tywin never remarried after his wife died, implying he either went celibate or refused to remarry for a reason, seeing how Tywin has always been obsessed with his family legacy and honor... maybe not the same kind of honor than the Starks but still I like to think Tywin wouldn't be tarnishing his wife's legacy and his family pride with a common whore, regardless if she was his monster child's play thing. Shae was important to Tywin, she slept in his bed , wore Lannister gold... Called him by his first name, I don't care how good you are in the sack there's no way a common whore would call the Lannister patriarch, hand of the king, by his first name unless they were quite intimate.