I am experiencing a mouth sore. I'm scared it could be herpes. I gave oral 1 day ago unprotected, but my partner says he doesn't have an STd?

Oral lesions. can have a variety of causes. It may not be a herpetic lesion. However, many people have been exposed to Herpes Simplex I during their lifetime. Your Herpes status can be easily determined with a medical test.
Probably not herpes. The first episode of oral HSV infection can cause multiple sores in the mouth, usually with a very painful sore throat as well. Also, one day is too soon for symptoms from a new herpes infection. Just one or two sores in the mouth usually are canker sores, not herpes. Recurrent oral herpes (cold sores, fever blisters) are generally on the lips outside the mouth. See doctor if still concerned.