My tongue frenulum is red and sore, what are some possible causes?

Local irritation?: It might just be irritated from some local irritation. However, without being able to see it, we can't be sure. Giving you a list of possibilities won't really help you in any meaningful way. Either wait it out for a few days and see if it goes away, or see a dentist to see if it is anything more serious than that.

Anatomy?: Sometimes the frenum is just a little too short. The fix for this is a very minor surgical procedure. Sometimes a habit, smoking, diet, etc can cause this. Have a dentist look at it to determine what should be done.