There is a simple physiological reason why your clothes are a bit more snug shortly after a workout: during the workout bout, circulatory blood gets redirected from other body systems into the working muscles, pumping them up. Also, circulatory blood goes near the skin surface, contributing to the pump. Further, you are probably hydrating during the workout and water can make you a bit pumped up as well.
Don't worry about it. It's a normal physiological reaction, and if your clothes weren't snug it would probably mean that you didn't work hard. Just give yourself a little while to shrink back down.
a-jock