I feel so sick 😞

Long story short, Ive been with my boyfriend for a little over a year now (I lost my virginity to him) and recently found out I have chlamydia. He says he didn’t sleep with anyone else and I am choosing to believe him. I have a doctors appointment soon and will ask her if the reasons he claims he got it is possible. He said: « maybe I got it from the steam room at the gym. » and « maybe you got a false negative the last time you got checked. » I’ve been getting tested regularly since we started sleeping with each other and every single one except the most recent one has been negative. He however has slept with 9+ girls before me and has never been tested, so it is quite possible. My doctor broke the news to me over the phone and then told me that in her opinion she thinks he’s cheated on me. It was a very hard phone call to take. I really hope he didn’t but right now it’s not looking too good for him. I’m taking Doxycycline and have been feeling EXTREMELY nauseous. I’m still in high school and attempted going to school today but as soon as I stepped foot in class I got a wave of nausea. I ended up texting my mom telling her to come pick me up and then threw up in the car. This medication is very aggressive and i don’t know how to make the nausea more tolerable. This is my grade 12 year and i can’t afford to miss any school so it’s upsetting that I had to today. Please, if anyone has been through something similar or would like to share some advice it would be much appreciated!