I gained weight after exercising? Help?

Okay I've never really worked out in my whole life before so I'm starting to exercise now. I worked out yesterday on the treadmill for 30min (running at 10km/hr and occasionally walking) and I was really tired after. Today I ran for about 15min and did another 10min of boxing. Before working out I weighed 52.6kg... now I weight 53kg! I don't get it.... aren't I supposed to lose weight? o_O am i doing anything wrong here ? Help! My aim is to lose 3 kgs (and get fit).