No, your steempower does not go down when you vote. Each vote is weighted based upon your steempower, but if we just let people vote an unlimited amount then we'd have nothing but a race to see who could vote the most the fastest. That's silly. So, each tim you vote your voting power goes down slightly, meaning that your next vote today will have less weight (will award less money) than the one before. Over time your voting power slowly retores back to 100% if you stop voting. You can check your current voting power by going to steemd.com/@brunotreves.