I currently have a canker sore on my gum, i found a good size knot under the mandible on right side. the knot is sore. should i see a doctor?

Yes: Yes you should see a doctor asap as this knot under your mandible is swollen lymph nodes that drain the oral cavity ares which has infection ( canker sore) , you need diagnosis and treatment ..

best to see Dr: Could be a a swollen gland, but always best to see a dr.

Preventative: Be preventative and proactive. Anytime there is a sore or a knot, i would see a dr. And help allay your doubts and prevent further issues if there is an area of concern.

Yes: Yes, not common to have "good size knot" under jaw from canker sore. Maybe not a canker sore. See oral surgeon.