Why does my body look worse the day after exercising?

I like to keep fit and go running in addition to strength training... this isnt too much of a concern but I have noticed on the day after doing cardio (running) my body loosens up and looks a little flabby around the belly and hips! however the day after my body tightens up again and I look myself.... is this just my body or has anyone else experience this?? any possible reasons? thankss... by the way I am not overweight or anything, I weigh 65 kgs and am 6 feet tall so am nearer the underweight bmi category than overweight category!!