singer was partying with pals at coachella music and arts festival over the weekend .
video posted on instagram appears to show the singer preparing a suspicious substance and then holding her nose .
speculation has been rife as to what exactly is going on in the video .
a comment on instagram , believed to be from rihanna , claims it was a joint .
this is not the first time rihanna has found herself mired in controversy at coachella .
in 2012 she posted a picture of herself ` cutting up ' a white powered substance on the top of a man 's head .