how much are you eating,
1 km swim burns about 320 calories, and then we can factor in increased metabolic rate for the next several hours due to that.
now the question then becomes what are you eating, how much, how often.
frequent smaller meals with a better composition of ingredients can help.
here is a calculator for you to help understand what you are burning
Calorie Calculator
you then have to figure out what you eat and how many calories are there. probably a good idea to make a journal of what u eat, how often, and when, as well as activity journal and see a nutritionist