Sore throat for 3 days. Dark green mucus. Hurts the right side of my chest when I cough. Stuffy then runny nose. Do you know what could cause this?

Seasonal flu. It is very common viral infection. Usually the condition resolves on its own with home care. Drink water and get a good rest. If there is no improvement or sore throat is severe or lasts longer than another week, or you are getting joint pain; see your pcp for evaluation.