I'm noticing red circular blotches on the side of my neck. No other side effects other than skin discoloration. What could it be?

Various - see a doc. Could be anything from a skin fungus, to an irregular blood vessel formation, to a skin symptom of a systemic illness (auto-immune, viral, etc.) to a medication reaction. Even if it doesn't hurt or itch, a new unusual mark on the skin should be examined by a doctor.
Depends on the cause. There are many things that may cause discoloration of the skin. Several types of inflammatory conditions of the skin, sun exposure, allergic reactions, bacterial, viral or fungal infections, insect bites, friction, and cancer can all cause discoloration of the skin. Have it evaluated by a professional and get the appropriate treatment.