Cunnilingus is one of the lowest risk sexual activities as far as STD's go. But you potentially can get anything you can get from regular unprotected sex.
In my opinion, I would say herpes is a concern. You can have oral herpes and not realize it and transmit it to the woman, or she can have it and transmit it to your mouth. One person can have it and not get many symptoms and say it's not a big deal, but the person they transmit it to can have bad symptoms and more outbreaks. And cold sores are oral herpes. It's harder, but herpes can be transmitted even when there is no outbreak because the virus sheds and can be on the skin. Some people just get a slight rash and don't realize it is herpes.
To protect yourself and others, make sure when you get tested for STD's that they actually do a "type specific blood test for herpes." This will detect if you have antibodies for type 1 or type 2 herpes meaning that you have been infected in the past. Many people get tested for STD's and don't realize they didn't have a herpes blood test. The doctor tells them they are clean when they actually carry the herpes virus. They spread it to other people because the doctors said they were clean. Make sure your partner is willing to get the same tests and show you a copy.