There are a lot of possibilities. You don't mention your age, or whether you're on any medications. Even supplements can cause a "flushing" reaction (niacin is a common culprit). Hormones and meds are the first thought, but it could also be your thyroid, blood pressure, or circulatory abnormalities.
Pre- or peri- menopause can start as early as your 30's, and can cause hot flashes, but usually these only last for short periods of time. If you're feeling this "heat" often or chronically, I think it's worth a call to your Dr. It's probably not serious, but easy to check out.