bad intermittent lightheadedness

Hey, something's been bothering me for the last few days. I went to New York City about a week ago, and while I was there I probably spent at least 10 hours on the subway cumulatively, plus another 20 on the train. Since I've gotten back I've had nothing short of bad intermittent motion sickness, especially when I'm not moving. If I'm standing still or sitting, it tends to flare up, but if I'm walking/running or driving, it's almost never there. I'm almost certain this has something to do with being on the train/subway for as long as I was, but I'm also a but of a carcinophobe, and the thought of a brain tumor is distressing. I wouldn't say that I have full blown vertigo, nor am I feeling particularly nauseous or vomiting. My appetite's been weird, but it's still mostly there. I would describe the feeling as more along the lines of lightheadedness. I feel as if I'm moving, but I'm not (and it's me that feels in motion, not the environment). I've been having tension headaches on random parts of my head, but that's probably because I haven't been hydrating myself too much. Could that also perpetuate my lightheaded feeling? What should I do about this? I just had a physical today and everything was fine. Should I go back and get some more specific things done?