This would not really be something that any type of std transmission would be involved in. Syphilis is transmitted in a sexual way from skin to skin contact but not from her to your back. Syphilis can infect your vagina, anus, penis, or scrotum, and sometimes your lips and mouth. So, her vagina touching your back in the off chance she has syphilis is not going to be likely to transmit. And then on to your mouth on her feet. She's not going to have a chancre on her foot. The main ways people get syphilis are from having vaginal sex and anal sex. It’s less common to get it from having oral sex, but it can happen. I would not worry about std's from your encounter. https://www.plannedparenthood.org/learn/stds-hiv-safer-sex/syphilis