I accidentally bit one of my plica fimbriata on the bottom of my tongue. it is swollen and red and painful. what should i do?

Nothing serious: I can understand your concern and frustration. However, traumatic lesion at the ventral surface of the tongue is a common problem and it makes you always anxious. The routine warm salt water rinse, local anesthetic application (e.g. ambesol), and NSAIDs (Advil) OTC every 6 hours will take care of the problem. If concerned, see your dentist for further investigation.