I'm sure this has been asked before, but I couldn't quite find it....so I figured I'd ask!

I am only trying to lose about 20 lbs I've gained during college. I've been working out for a few weeks now, but I haven't noticed much of a change in my weight. I am doing cardio and some stregnth training to work on my abs. Am I not noticing a difference, because I went from such a lazy lifestyle, to suddenly working out and eating better? Or am I gaining muscle, and that's why I feel like I'm not losing anything? Any answers to this would be helpful. It's just confusing me, because last summer I worked out and lost weight very steadily and regularly. Thanks!