Constantly Lightheaded - Is This Heart, Anxiety, Or Vertigo?

I am 24 and female. Lately, I have been getting lightheaded when I get out of bed in the morning, or lay down. Occasionally, if I am lying in bed reading or watching TV I will feel disoriented or lightheaded, near greying-out. I put my hand above my head, like putting my hair up in a bun, I feel dizzy. A few times, my friends or It's Always Sunny make me laugh really hard, I feel lightheaded. It is winter in New England and when I go indoors from walking outside, I feel like I could fall over. This also sometimes occurs in the bath or shower. If I am laying down and I adjust my neck, sometimes I get what feels like a headrush. I have minor jaw pain, that comes and goes, and is probably unrelated. I am overweight (I have always struggled to lose weight), but I am in good shape. I ran a half-marathon a few months ago. This isn't constant, I would say this happens about 60-70% of the time. I feel better when I push fluids, but it is markedly worse when I am menstruating. It's not debilitating, but it is worrisome.

I went to see a nurse practitioner and she ran a few blood tests - iron, metabolic, and thyroid - they all came back normal. She also listened to my heart, and told me that sounded normal. My blood pressure is typically in the 103/61-64 range. The NP wants me to see an ENT, but I do not think it is vertigo.

(I have always had strange bouts of medical mysteries that have all be chopped up to anxiety. As a child, from about nine to ten, my earlobes, throat, and feet would swell for seemingly no reason. At seventeen, my left pupil would hyper-dilated. Never figured out why any of that was happening. Last summer, I was admitted to the ER for having chest and left arm pain. That was determined to be muscle spasms.)

Does this sound like anxiety, or something else? I'm sure it is nothing, but my mother is a nurse and she is decidedly concerned. Thanks for any help.