I got it a fair bit for the first year or so when I started lifting particularly on heavy squat and dead sets, I too considered myself quite fit from playing soccer. I guess its a different form of conditioning, and it should lessen over time. Especially as you are just starting out, your body needs to do a lot of adapting to the stresses of weight training, give it time, and stick with it. I still get it from time to time, mostly doing heavy 5 or 20 rep squats. I believe it is a combination of blood pressure change, breathing (or lack thereof, it is really hard to take full breaths towards the end of a set, under a heavy bar) and fatigue during sets, as well as movement of blood around the body.