I've had it stuck in my head for a few days, I can't get it out. I know I never read an ending so I don't know if it was finished or not. I do know it was a Sess/Kag.

From what I remember, Kagome had gone over to her bf Inuyasha's place to tell him she was pregnant and found him in bed with her friend (they didn't notice her), I think it was Sango but I'm not sure. So she left and was upset and was walking over to another friends house and is nearly hit (bumped her I think) by Sesshoumaru's car. He offers her a ride as an apology. I know she was masking her scent with her miko powers so nobody would know she was preg. I remember she was upset thinking about how her family would give her the boot for getting pregnant while unmarried.
He was expected to go up to his father's place for some kind of family time or something. He didn't want to go alone because he had been telling his father how great his girlfriend was (Kagura) when really it wasn't and they had just broken up (he kicked her out more like) and so instead of simply fessing up he asks Kagome if she would masquerade as his girlfriend for however long the family thing was (because they had never met her and his gf had always been referred to as K-chan). He offered to pay her, I know she talked him down to less because she was uncomfortable with the large amount of money he offered.
She doesn't know Inuyasha is his brother and when they arrive at his father's house they find out his brother isn't coming or something. And thats where my memory leaves off.

So there ya go. I'm glad I remember so many details, hopefully someone will know this fic too.

i believe that fic is, Liar, Liar » by R. M. Avalon www.fanfiction.net/s/6622922/1/bLiar_b_bLiar_b
When Kagome signed on to play the role of Sesshomaru's girlfriend for a week, she had no idea what she was getting herself into. Everyone is lying about something and someone is bound to get hurt. SessKag
Inuyasha - Rated: M - English - Romance/Drama - Chapters: 4 - Words: 23,198 - Reviews: 124 - Updated: 3-26-11 - Published: 1-4-11 - Sesshomaru & Kagome

That is exactly the one, thanks R0o.