Working hard! buy still gaining... why?

Ughhh.. lol I am beyond frustrated and a little discouraged. It has been a month since I decided to improve my life. I am eating healthier, running at least 20 minutes a day 4 or 5 days a week with some other work outs. I went down a size in pants, I feel happier and stronger.. but I have gained 3 pounds?? lol please help me!