Unfortunately, it IS possible to get bigger with exercise if you don't lose any fat. Any muscle you build (and Baff is right, it takes a long time to add a few pounds of muscle) will push out the existing fat and make you bigger.
That's why it's essential to lose fat while you build muscle. And you lose fat through cardio and controlling calories. I know you said you're doing cardio 5 - 6 days a week -- are you doing it long and intensely enough to really burn some calories?
And how about your eating plan? Exercise can increase your appetite and it's easy to overeat after a tough workout if we're not careful. Do you track your calories and weigh and measure portions? Do you think you might be eating more than you did before you started going to the gym?
It's awesome that you've added the gym to your life and I can guarantee it will only help your weight loss and maintenance in the long run! Bit I know you don't want tight jeans in the short run, so maybe consider your eating and cardio. If those are A-OK, then yeah, it's got to be water retention, which will pass.