GuitarRox is right - there is absolutely no risk for STDs, including HIV, from this. Most die quickly once exposed to air, and some require direct genital skin-to-skin (or mouth to genital) contact to transmit.
Unless you had some kind of sexual activity at this party, you aren't at risk for any STDs.
An important part of STD transmission is the sexual part. While that is a germy area, falling into a toilet is not going to result in std/herpes transmission. I wouldn't worry about this and take a nice shower and wash up. You should be fine.