I keep getting cold sores back to back I saw one coming up so I used the abreva it went down n couple days later it came bk up what is causing that?
Herpes virus. Cold sores are viral infections and the virus "hides" in the nerves close to where the ulcer is. That is why they tend to come back in the same location. The best treatment involves antiviral meds. There is a topical application called viroxin that kills the virus and very effective at the early signs. However if your immune system is low, they come right back.
See your doctor. This problem can only be solved by seeing your doctor and being evaluated face-to-face. After a thorough examination, your doctor should be able to tell you what's wrong and what to do about it.