The tattoo was noticed weeks after Payne's girlfriend paid a touching tribute to the late singer and their relationship.
Cassidy shared a montage of precious moments from her two-year relationship with the former One Direction singer in the now-deleted video on TikTok.
In the caption of the compilation video, she wrote, "I love you", against the backdrop of the song, Fade Into You, by Mazzy Star. Cassidy also included more mundane moments with Payne as they lounged on the couch together and the late singer snuggled up next to her in bed.
Flashes of their travels together, bowling dates, glamorous events they attended and a sweet note Payne wrote her were also included in the montage.
The Instagram influencer also previously claimed Payne was the "love of her life" and that they had planned to marry the following year.