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I'm not sure, but it might be burning oil.

Check for antifreeze in oil?
Has it overheated at all?
You can get a tester at any autoparts store to test coolant for any oil in it. Usually if you have oil in the coolant (and not the other way around) it will build up extra coolant pressure. Either of these issues means replacing the HG and machine the head. Hopefully its something simple like a sparkplug worked its way loose or something.