hello, can anyone help me with information on the STD herpes?

I have recently been diagnosed and I am shocked and confused. My boyfriend and I lost our virginities to eachother and have never cheated.

We broke up for a month and got back together and after we had sex I noticed bumps which turned to painful blisters with itch. The doctor said it is herpes and gave me medication.

I know herpes can form from a cold sore, but I thought considering we are eachothers only sexual partners the chances of STDS would be low.

Also we have been together for a year and a half so why is it only showing up now? And my boyfriend has no symptoms whatsoever.