The transvaginal ultrasound would not transmit an STD. Most of the time, they have covers for them that they use once for each patient, or it's disinfected between patients, and STDs die quickly once exposed to air, so even if they didn't do either of those things, you wouldn't be infected.
How long have you been with your partner? If it's been a short time, it's possible that he got an STD prior to being with you. If it's been years, he cheated.
I do wonder about his negative tests if you both had symptoms. Has he been treated for anything? Have you seen his tests?
What were you treated with? Do you know the name of the medication? Was it taken once? For 7 days? Was it a shot?
If you do test, it will be hard to rely on your results right now since you took the meds. I'd love to know the names of them, though.