Always check any possibility of physical illness. One of the most common causes of a reaction like this, especially in someone around your age up to 18, are blood sugar issues. If there is a history of diabetes in either side of your family, chances are you could be developing some sort of a problem and this is the first warning sign of it. Regardless of what people say, you know when you're stressed because you can feel it, your body just doesn't suddenly do something like that because you're stressed. Suddenly reactions like that where you're sure you weren't stressed out or anything are physical, something is going on. Has this ever happened before? The light headed feeling is a dead give away that likely you are starting to get a blood sugar problem. Catch it now and be sure if you have it or not and you can easily correct it. Do you have any other symptoms through the day? Sleepiness that seems odd, odd fainting spells, confusion, anything? When I was your age I had the
same thing happen to me, only to find out years later, and after getting a horde of symptoms you don't want to experience, that it was simply a blood sugar problem easily corrected with the proper diet.