Could I have an STD?

My vagina has been itching really bad lately! Like the inside of it! Especially after intercourse! I first thought it was a yeast infection but now I have yellowish/greenish colored discharge and starting to think it might be Trichomoniasis but it says it's spread through sexual contact. Me and my fiancé have been together for 3 1/2 years and I was tested for diseases a few months after we got together and j was healthy and he says he's never cheat on me and never will bc he knows how it feels and he's not like that. Could I have something ? Could he have been cheating on me and got something? Help! I'm scared and freaking out!