Why do I get so cold sometimes?

Sometimes I get cold for no apparent reason. Doctor says my iron levels are very good, and I don't use any drugs that would make me feel cold =P lol but sometimes I just feel freezing. I weigh 200 pounds. For a 5'10" guy like me, that's a *tiny bit* fluffy, so it can't be from being underweight. But it's summer, and I still have to wear thick jackets sometimes! Any ideas as to what it could be?