Are you sure she was masturbating? The substance she has on her hand looks more like semen than vaginal fluid. The look on her face isn't that of pleasure, which seems to support the theory that some guy just came inside of her and she's trying to clean up.
The tissue box doesn't really mean much, as they're generally on hand for sex as well as masturbation. To be honest, it does look more like the aftereffects of sex than masturbation, but if the artist so tagged it...