STI's & Symptoms

I am not going to beat around the bush here ladies & possibly gentlemen. I have been having a decent amount of sex with my boyfriend, & all of it unprotected because heat of the moment (yes I know I should be using protection, I just forget to). So in my highschool years, we were taught if you have sex for the first time with a new partner to go into the hospital for STI testing, so I did. The doctor I saw told me not to worry about testing, and that if I started to show symptoms to come back & that THEN he would do a test for me. So my question is, is it possible to have an STI and not show symptoms? Or is it, you know you have a possible STI of there is symptoms? All help & comments are truly appreciated, TYSM!