Knot under the chin that causes pain. not in the mouth, or a lymphoid?

Go to Dr: Difficult to know. Someone needs to examine you. Is it an enlarged salivary gland, cyst, nodule, or lymph node?

Get it checked: Like you said it is a knot. We can't really tell for sure tll it is checked. You might get lymph nodes that could be tender especially if you had a cut while shaving or have a sore throat..Etc.. However it is unusual to have one single node only. I would get it checked to rule out other problems including inflammed/infected cyst for example.

Painful Knot Chin: A painful "knot" under the chin is either a swollen, inflammed lymph node or a swollen salivary gland, the submandibular salivary gland. Unless you have associated fever, sore throat, night sweats or weight loss, give the "knot" a couple weeks to resolve. If it doesn't disappear on its own, go see your doctor for an exam.