STI need advice

So I just found out I have a STI but my bf thinks it’s all me. I got tested after the 1st time we had sex n I came out clean. I got tested awhile back n it came back that I have a STI n he told me he had protected sex with this girl after he got tested n March n he came back clean after he got tested. But he never got tested after he had protected sex wit that girl. Wht is the reason for this