2 main possible reasons in your case:
You may have low blood pressure and the hot water will widen blood vessels in your whole body, effectively removing part of the blood from your brain.
You may suffer from gestational diabetes (this condition is actually not that rare) and after not eating for a long period of time - your blood sugar is too low.
If this happens, you will also experience trembling hands, numb lips, vertigo and hunger for carbs and junk food.
First condition is not life threating, you may live with low BP for 100 years (well, there will be headaches and light headedness)
The second condition is dangerous and you better visit your doc.