My initial point is that she initiated the changing room sex and she did, she was the one that invited the MC in and she was the one that asked him to fuck her. Period. It is in the scene. It is a fact.
She was just putting on a front because that's who she was coerced into being by her parents. She is supposed to be serious, prim and proper. She wasn't 'legit' shocked because if she was, she would have put up an ACTUAL front and tried to find excuses instead of going with the flow like she have done multiple times during the game and, even so, we all know because of Jill's own internal monologue during ep6 (I believe it was ep6... maybe ep7) that she does it out of insecurity. So this was all basically a fake front she was putting on like she has done before.
And if you need proof, here you go. The MC was ready to back way after gropping her same as he has done other times, but this time Jill was honest with what she wanted to do: