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.

I think it's that post-orgasm tiredness. Plus the tissue box is there. And are you by any chance insinuating that women don't drive pleasure from sex?

The artist tagged it as masturbation.

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...