Running Hot burning coolant

For the past month or so, My car is running hotter then normally but mostly when idling. I have replace about a jug and a half of coolant in the last 3 weeks. My check engine light came on around the same time. My motor oil is not milky, and there isn't any white smoke coming from the tailpipe. I do occassionally smell coolant burning. Any ideas what this could be? And what would be the easiest way to find out what this is.