This is totally normal, not preventable, and a regular part of exercise. www.sparkpeople.com/community/ask_the_expe
Just know that you are burning fat, and your body is improving. The scale is just about the worst measure of our progress that we have. All it shows is gravity's grip on us that day. It doesn't show our body fat percentage, nor our energy, the health of our body, or anything else.
Don't worry too much about it at this point. :)