So first, assuming she has HIV is a stretch, as most people don't.
Second, HIV isn't found in saliva. It's found in blood, semen, vaginal secretions and breast milk. Even if her gums were bleeding, saliva has an enzyme that deactivates HIV, and there wouldn't be enough blood to transmit the virus to you. So HIV isn't a risk.
No other STDs are transmitted this way. To get an STD, you need to have some kind of sex - oral sex, vaginal sex or anal sex. You can also get it from naked genital to genital or genital to anal rubbing or grinding. None of this happened.
There is never a risk from getting a hickey.