I have braces and I have a sore tooth the dentist said it isn't a cavity so what else could be causing this?

Braces& sore teeth. Braces pull teeth through bone to a better position. This causes the tissues holding the teeth in to become sore. The sore part is adjacent to the tooth and can feel like a toothache.
Normal, ? When teeth are moved Orthodontically, they become sore. The duration and extent is perceived differently by different individuals. It could also be other things such as gum issue or bruxism but that cannot be determined over the internet, sight unseen. You can run this by your General Dentist as well.
Soreness. If I gently pushed on your thumb you would say, "That's OK." If I pushed continually for a few days you would say, "That hurts." Tooth movement and changes in occlusion require expenditure of energy. The energy cycle, the Kreb's Cycle, produces a chemical called Adenosine Di-Phosphate, which is the chemical that makes pain nerve-endings fire. Take Advil, (ibuprofen) eat high protein diet, salt water rinse.