I'm in a serious medical issue / cheating boyfriend?

Olivia
I have never had a yeast infection, only two UTIs ever... never had any stds. Tested regularly. I got tested before my iud was inserted (skyla). Everything was good. Insert d last days of period.. no pain. Then a week later me and my boyfriend have sex in a hotel. We've been "faithful" for two years. He came in me for the first time. I start bleeding and then all this pain in my abdomen follows. Bleeding doesn't stop. Pain gets worse and worse. I go in and my dr checks iud. Looks good. She does std test. Comes back with TRICHOMONIASIS. 
Wtfffffffff I NEVER CHEATED. DID HE CHEAT! She said it's only transmitted through sex but u really don't believe that. Like can it be given any other way? Could he really not of cheated? Could it of came from hotel? Since that was the first time he ejaculated does that mean he could of had it but I never caught it until now?