I hooked up with a girl last night and had non-penetrative sex. We rubbed each other's body for half hour. At some point my fingers and my genitals were very close to her genital, and also stroked my genitals (with hands) for a bit, but did not penetrate nor get inside her. Eventually I ejaculated on her chest. Am I at risk? If so, what kind of STD should I be worry about? Do I need PEP treatment for HIV prevention? Thank you. 

The encounter you describe was a no risk event for nearly all STIs, including HIV.    BOdy rubbing (frottage) and digital masturbation are no risk for acquisition of HIV, Gonorrhea, chlamydia, trichamonas and most other STIs.  This is the case even when partners get each other’s genital secretions on each other.  PEP is absolutely NOT needed.  There is a very, very small, theoretical risk for acquisition of so-called “lesion” diseases such as Syphilis, herpes or HPV (if unvaccinated) but the risk for these infections is truly negligible.  Nothing you describe suggests any need for concern, for testing of any sort, or treatment.---