There are a couple reasons that may cause this, and I’d recommend you consult with your gynecologist. Vaginal intercourse introduces microbes from your partner’s genitals into your vaginal ecosystem, which can cause a disruption in your pH and microbiome. It could be that you and/or your partner have a chronic yeast infection that hasn’t completely healed. It could also be that you have chronic dysbiosis and are in need of some microbiome support, via probiotics, diet, and immune support.
You can consider abstaining from sex until your yeast infection has completely cleared, while you step up your microbiome-supportive health rituals. It is also important to maintain adequate hygiene: both partners should clean their genitals after sex, practice caution with lubricants which may irritate your skin, wear looser clothing and undergarments, and consider using a non-irritating barrier method.