Well, if she's in a LTR, that's probably a red flag right there. Why she was flirting in the first place is anybody's guess.. However, beyond that, and don't take this the wrong way but how open are you about your fetish? In other words, is it a known thing regarding your liking feet and tickling?
If the answer is yes, then the possibility exists that this girl was fucking with you in some sadistic manner, and if that's the case then others are in on it too. Don't want you to get all wound up but in today's day you almost have to be suspect of everything.
You mention everybody was drunk and stoned, so maybe not. The possibility surely exists that you were extremely caught off guard (and no one would blame you) and you blew a good opportunity.
Not wishful thinking, there was something going on there bro. It may or may not come up again but proceed with caution as the conditions are highly unlikely to be exactly the same again, unless you guys party like that frequently.