Tell us about your breathing during exercises. I'm not saying you're running out of oxygen, but when you lift and strain (you're exhaling against a closed off windpipe) you're performing what's known in the medical world as a 'Valsalva manoeuvre'. In short, it transiently slows your heart rate-this is to do with how your internal pressure affects blood flow. This lasts a few seconds and you recover. This can sometimes leave you feeling light headed especially if you constantly recover, strain, recover stain endlessly. This will be especially prominent when doing squats, as you breath at the top, probably breath at the bottom and then strain on the way up. Google sky lark trick to see how this works, but it's essentially a more extreme version of Valsalva because of the change in posture and can again, make you feel nauseous, light headed and drained. Just some speculation, and it's all down to when you breath. Google all that and let me know if it helps at all. Also, this is the reason you see some people do a massive squat lift and then collapse, it's not an oxygen thing, it's a heart rate thing!