recently changed girlfriend and started having sex with her 4 weeks ago.

First week of sex was fine.

second week started feeling a slight burn on testicles but didn't pay much attention and noticed i started to loose erections.

Third week of sex i just could not keep an erection .

Was prescribed doxycycline day 3 now still no improvement .

The symptoms are as follows :

Burning ,aching sensation on balls

burning sensation on penis, although pass urine fine.

Very hard to get an erection ,can't hold an erection.

Today had a chat with her about it and she said about 10months ago she had a yeast infection which went on it's own and didn't see a doctor .

What's the reason for this ?