While it is not super common (oral herpes/mouth herpes to transfer to genitals), it can happen. The only way to know if it is herpes is to get it swabbed. But it has been four weeks and that is a bit long. Most develop pain and itching for 2 to 20 days after infection and sores 1 to 2 days after that. It's been longer for you. But if you are worried, get it swabbed and tested.