Fingers don't transmit STDs the way a sex toy might. Sex toys are often made of materials that can hold on to fluids in a way that fingers don't.
Sex toys aren't all that efficient in transmitting STDs, either, but it's more likely that you'd take the toy from one person to the next without a lot of things in between. Even if the CSW fingered herself, then fingered you, in between that, she probably changed positions, you probably did as well, she probably used her hand to help her shift, and touched the bed or table or whatever surface you were on, etc.
Here are some posts from our experts (when we had them) that may help -
https://
www.medhelp.org/posts/STDs/Nervous-about-mutual-masturbation/show/2183476
https://
www.medhelp.org/posts/STDs/Mutual-Masturbation-STD/show/1968429
https://
www.medhelp.org/posts/STDs/mutual-masturbation-sti/show/1858544
https://
www.medhelp.org/posts/STDs/Understanding-the-risks-of-mutual-fingering/show/1516796
https://
www.medhelp.org/posts/HIV---Prevention/Vaginal-fluid-on-hand-touched-inside-of-condom-and-head-of-penis/show/1119533 - read this thread thoroughly. There is an excellent explanation about why certain activities are not risks, and why others are.