It may have been the pH balance of your partner and/or if you had not washed yourself after each encounter as well. You can try some Hydrocortisone (1%) cream. Which is over the counter and also keep as clean as possible as well. It will take a bit of time to heal. If it becomes worse or if you get other symptoms then do see your doctor, you may need a prescription strength.
Hope this helps, If you need anything else please feel free to ask. If you found this helpful don’t forget to click the *accept* button!