What could’ve happened ??

My boyfriend of 5 months & I have been dating for over a year... we finally got intimate & like 1-2 weeks later he notices std symptoms. Then gets tested & it turns out positive & now I’m positive. Apparently I’m the one that was carrying a std from my ex.. every other week he finds a way to bring it up. Today it got to the point where he kept saying it questioning me with a hateful voice as if he regret dealing with me so I hung up & he kept calling me, so I blocked him.. (since he says he’s never had an std until me)

But, when I was with my ex I would always get tested & it always came out negative for everything.... I’m not too sure what could’ve happened..