What Causes On And Off Burning Sensation In Vagina While Having Sex?

Hi. I m having some issues down there. I ve had four partners. With two of them my vagina burned when coming in contact with their semen. The other two, I was fine with. I ve been with my boyfriend for about four months. We ve had plenty of sex and it never burned. He stopped smoking last wednesday and on thursday night, I was burning. Again on Saturday night. The only common denominator was my other partner (the unburning one) also smoked. Does this have to do with my pH system....or....? Heeeeelp! I do not have any stds and neither does my partner.