My s/o and i recently had anal sex, and accidentally inserted some of his penis is my vagina right after. what can i do to prevent infection?

Wash: Just do things to keep yourself healthy: wash, drink a lot of water, sleep and don't stress. Take probiotics like acidophilus to support the healthy bacteria in your body: yogurt, apple cider vinegar, sauerkraut, kombucha.... I can't recommend douching because it can push bacteria further up into your system and increase risk of infection. However you can wash well with a gentle soap.