How Do I Know If I Have STD? I Had

how do I know if I have STD? I had sex with someone and he talked to me earlier and asked me if I have STD. Apparently, soon after we had sex, he developed blisters in his groin and suffered from UTI. I only had sex with 3 people prior him. All of those are exclusive partners at the time I had sex with them. Now I am afraid if I really caused him STD and if I have one. Please help