After i had sex and he cam in me i have a yeast infection why does that happen every time he does only in me not on it?

PH change: The pH level is a measure of the acidity or alkalinity of a substance. The vaginal pH is kept at between 4.5 to 5.5 on a 0 to 14 scale, which is slightly acidic. Semen is alkaline with a much higher pH and often displaces the "good" vaginal bacteria that keeps the pH at the previous level. This acid/base reaction can raise the pH to alkaline, which yeast love. Diabetes & douches do the same.