This is basically inflammation being manifested through the skin. It feels warm to the touch, and the articulation may increase in size slightly and become tender. This happens because inflammation increases the blood flow to the area, which causes redness. The increased blood flow is met by the same outflow of blood through the veins, and the excess liquid stays in the tissue and makes it swell.
The inflamed tissues stretch because they are filled with fluid, and this stretching of the tissue stimulates nerve terminals, which send the pain signal to the brain. That’s the main reason why inflammation hurts, and this symptom is more commonly found in arthritis triggered by systemic inflammation, such as rheumatoid arthritis.