What Causes Red Rash On Scalp,crown And Swollen Lymph Nodes?

I have red rash on my scalp, near the crown, associated with swollen lymph glands behind both ears. I am having dental work (root canal in stages on upper molars, both sides) performed since this past weekend. The redness of the scalp has become tender, itchy and flaky within the last couple of days. Since Sunday, the first day of treatment of one tooth, the lymph nodes swelling in the neck has decreased but not entirely disappeared. On Sunday, I had bloodwork done at an emergency hospital and the physician could not see any infection. Thank you for your time.