Cheater help

Last year I was hooking up with this guy for about 6 months before he left for studying abroad. We had both decided to have unprotected sex and made sure to get tested after and we were both clean. He left for studying abroad for about 3 months and came back. We had hooked up again without protection and reassured me he was clean- after he kinda ghosted me and I found out he’s had a girlfriend of 3 years. I haven’t seen him since January and now I’m having symptoms of an STI. I just got tested and it’s confirmed- he’s the only person I’ve been with. I’m not sure if I should dm his girlfriend and tell her because I’m not 100% certain but pretty sure it was him. What do I do?