Giving head (i.e. performing oral sex, giving a blow job) can put you at risk for chlamydia, gonorrhea, herpes, syphilis and possibly HPV. Getting HIV from giving head is extremely rare. 

It can take several days or up to several months to show symptoms of STIs. STIs in the back of the throat often have no symptoms and some clear on their own. A throat swab for gonorrhea and chlamydia and blood test for syphilis can tell if you have an infection. Because most STIs have no symptoms, the only way to know for sure is to get tested.  Sexually active gay men should get screened for STIs and HIV every 3 months If you're concerned about your HIV risk, talk to your provider about whether PrEP (pre-exposure prophylaxis) makes sense for you.