I recently had a new partner whom I had sex .?
It probably means that your new partner has some strain of bacteria growing in her vagina that her body can tolerate, but your new partners body cant. Different bodies have different reactions to the same strains of bacteria, depending upon each persons individual genetics and also upon the mixture of species that are already growing in that area of the body. If you are lucky, then your old partners vaginal flora may eventually settle down into an equilibrium that includes the new bacterial species. But if not, then the problem will continue to occur whenever you have sex with both partners in succession without using condoms.