Just like your gut, your vagina is home to all sorts of bacteria, says Dr. Streicher. The "good" kind, called lactobacilli, help keep vaginal pH between and . But if the vagina's pH is disrupted—say, from semen or menstrual blood—it may become less hospitable to lactobacilli and more welcoming to bad bacteria. The overgrowth often comes with a fishy odor as well as vaginal discharge and irritation. For women who notice a slight smell after sex or their period, using a pH balancing gel, like RepHresh, can make the vagina more acidic and help lactobacilli get the upper hand again. That said, if the bacterial imbalance turns into full-on BV (think: painful urination, itching, more discharge and odor), see your doc for an evaluation and treatment (possibly antibiotics).