Fooling around with Brittani in that exact moment is basically taking advantage of her being depressed and vulnerable. It's not a big surprise she'd feel bad after that
It's actually something that makes relatively little sense to me, given the setup and what we've seen of Brittani so far.
* during the first interaction with Brittani, we can see that she doesn't exactly get hung up over her previous choices that could be considered mistakes -- when asked if she'd break up with Jamie now that she knows he was still with Gabby when Brittani and Jamie hooked up first, she's basically "no, what's done is done and it wouldn't change anything".
* when Guy wakes up after the punch, Brittani explicitly tells him that at this point she considers herself single, because per her words, she doesn't want to date guys who get violent
* then, after fairly long consideration, she makes a decision to make a move on Guy. She does not show any signs of doing it out of being depressed, but out of gratitude for how Guy has acted towards her, which was, again per her own words, something no one has done for her before
And then i'm supposed to suddenly believe this girl who already mentally kicked her shitty excuse of a boyfriend to a curb and who up to this point displayed nothing but high emotional resilience, is suddenly completely devastated because the very same guy she, per her own statement, didn't want to have anything to do with anymore, who scared the shit out of her with his violence, who is a known cheater himself --given he was more than ready to hook up with her while still together with Gabby-- and who was her boyfriend for whole 1-2 months tops, had the gall to call her a "whore" for making a move on someone else after he got already dumped? And stormed out as if
he was wronged here? Honestly, it'd be much more believable if she dished him out more of that Goth Brittani treatment in return, for all the gall and stupidity he's expressed throughout. Because Jamie is absolutely in no position to cast any sort of moral judgments, and Brittani is more than enough smart to know this, or let it get to her like that given the actual circumstances.
This reaction we see instead, this feels like something you'd far sooner see from Gabby, given how both sensitive and explosive she's shown herself be.