Why am I gaining weight?

Firstly, cardio works out your muscles, if it didn't then you wouldn't be able to move as you use muscles to move, it just doesn't work out a muscle specifically so you could have got muscle gain here. You stupidly didn't mention your age, because you could be having a growth spurt. Most of all and this is something that everyone seems to forget, DONT WORRY ABOUT WEIGHT. People always say ' I want to loose weight ', why? You want to loose body fat, not weight, one of the most heaviest blokes on earth is one of the most muscular who has hardly any fat on him at all, but would you think he is fat? If you are trying to loose fat, then carry on with your program, it has to show results eventually, if not it means you are not human. Oh one more thing, instead of measuring your weight and then measuring it again later on at a random time, take an average, weigh yourself every day, write this down, make sure you measure yourself at the same time of the day for a month, once you have your results, find the average weight but adding up the amount of results then dividing that number by how many results there are, this is far more accurate.