My friend is being cheated on... ish?

a couple weeks back me and a few friends decided to go to the local park with a few drinks, kinda like an outdoor party ( we aint drunks, we rarely drink more than like once a year) , the name of an old friend i hav'nt seen in years was mentioned and i decided to envite her to the gathering too, she a friend since i was like 3, but going to different secondary schools kinda seperated us, anyhoo she came we all got drunk, and we started to kiss etc, at the end of the night she invited me back to her place ( well more like told everyone im stayin there or else lol) . i realised that this could be a big mistake and declined, she got pissed off and walked away, by the end of the night we wer friends again ( we all talking as we walk everyone home) . the next day, being immune to hang overs we all went out again, me and her seemed to both be pretending we cdnt remember the previos night, anyway she took a shine to my friend and from that day they started dating, now we wer all back in each otheres lives now so i and my friends started going to her house often ,like i did wen we wer kids, but i noticed that when its just me her , and her friends ( non of the party go-ers) she tends to use behaviour im not sure she shud do. lets say we sitin down watching a movie, she wud sit right next to me , huggin me etc, sometimes kissing me on the cheek, yno, general handsy behaviour, now she used to do this when we wer kids ( which i ddnt like, she had a crush on me back then ) , so at first i never really noticed, then one day i thought to myself, if my girlfriend was doing this with another guy i would be P++++D! so im asking, what are your views on this situation , and what should i do, i'm not desperate for help, but i tend to think outside the box in life, and want regular opinions to judge my next action