I think she was sad because she knew she had made a mistake by sleeping with Chuck, she knew Chuck still loved Blair anyway

Well obviously.

It's not like she really liked Chuck or anything, but how could you not feel bad if something like that happened?

I think Chuck forgetting her was the least of her worries, she was probably more scared about anyone finding out rather than him remembering her.

And besides what did she expect?

but she sould tought about that, he dont care about her in the same way as he do for Blair

I think that's when she realized what a mistake it was. Chuck obviously was in love with someone else and she had sex with him for all the wrong reasons - because she didn't want to be alone and neither did Chuck. But Chuck has someone who he loves and that person loves him back and Jenny doesn't have that.

GIRL,like for real..what did you expect?

That's Proabely Why She Said Even Chuck Hates Her:(