I like to knit by a window or while 'watching' a show I am kind of interested in. It gives me a chance to take a break from my knitting and focus on the things around me...I don't get nauseous, but I do tense up and need a bit of a break to stretch out a little and get my eyes moving around again...lol...Do you hold your work really close to your face while you knit? It could, as the others have said, indeed be eyestrain, or a mix of overall body tension and eyestrain. I'd say if nothing else, keep a glass of water close by, maybe some crackers, and make sure you're taking several small breaks while knitting. Not necessarily getting up, but, change the way you're sitting, stretch out, look around a room or out a window. You may also want to crack a window, if you knit near one, just to get some fresh air.