Gums in one part of my mouth are red and inflamed. Is this gum disease? What should I do to treat this?

It could be... .. but it needs to be examined to know for sure. You may have food or calculus stuck there, there may be a cavity, but at a minimum it is inflamed. To determine what causd it and what treatment is needed, please see a local dentist.
Gingivitis treatemnt. Red, bleeding and swollen gums can be a sign that you are at risk for, or already have, advanced gum disease. Untreated gingivitis can lead to the tooth loss and other health problems. See your dentist ASAP because the sooner you treat it the better the outcome.