Last week I tested negative for HIV. If it's been eight months already and I haven't shown any symptoms whatsoever (I'm male and I've been keeping a very close eye out) what are the chances that I still have a completely symptomless STD?
well you can have herpes, often it lies dormant and then you'll have the breakouts.
Get screened anyway.
This really depends some on the type of sex act involved. you could have been exposed to Hepatitis-C or HPV or something else and not realize it. The only way to know is to get a full STD screen. The type of test you took also has some bering on how soon you may really know the long term truth. also keep in mind that some of these "quick tests" may show false positives if you are using certain drugs. so if you have a positive it is important to verify the positive results with a Western Blot blood test to confirm a positive result. Use condoms for everything, use a new one for each sex act buy flavored ones or flavored lubricants. If you do this and get a full STD screen once a year you will sleep better and minimize your risk of an unplanned pregnancy.