What Causes Weakness And Fainting Spells?

Hi. I have an 18 year old neice who is experiencing unexplained and severe episodes of weakness and lightheadedness. She has fainted once. It seems to happen most often after she eats, which makes me think it's blood-sugar related. But, there is no history of diabetes. And, she's had a standard panel to rule out food alergies which was negative. Her doctor did check thyroid function, which showed on the low end of normal. She has coincidentally been getting hives for the first time in her life, but I can't see how that could be related. She has a long history of severe allergy induced asthma (cats mostly), and is on strong medications to keep that in check.

My question in this: She has lived all her life in a house 50 meters from a major high voltage power line. EMF radiation from screens and microwave radation from cell phones have also been no small part of her life. Her mother has recently had problems with thyroid function (including several surgeries, but not cancer). Her parents don't think that EMF radiation is an issue. But, in your opinion does this put my niece at a higher risk for any particular disease? Is there something that I should have her doctor look for, given the proximity to the power line? (Her doctor is frankly puzzled and at a loss at this point.)

Thank you.
Jim