infection from sex??

It seems like every time my boyfriend and I have sex I get some type of vaginal infection. Wether it be a bacterial or yeast infection. I live in Florida and I assume it's from the hot weather and Victoria Secret underwear aren't the best to let it breath.. so I'm going to be switching to 100% cotton underwear. But could I be getting these infections from sex? I have gotten 4-5 within the past 2 months. 
I was also thinking is it from him using his spit to make things wetter?.. tmi ik but I just don't what else I could be getting it from.
Update!!
Thank you for the replies 
I do pee after sex 
But I will tell him from now on we both should wash up before sex and also we're both each other's first. 
Is there any lube recommendations?