The skin inside my mouth behind my top lip is super sensitive and kind of sore and seems like salt really bothers it. what could cause that?

It is very likely that you have a small ulcer on the inner side of your lip. Such ulcers are quite common affecting the mucosa lining of the mouth. Sometimes these are caused by virus infection. They usually subside in 4-5 days. You can take Vitamin-C tablet once or twice daily for 4-5 days and the pain should go away. If the pain is persisting beyond 1 week, you should show it to your dentist.