Its possible you're gaining muscle faster than burning fat, in fact, that will almost always happen when starting a new workout regime after not having done much before. Depending on the weight lifting exercises, you may want to tone down on the weights, be it less often or lighter weights and focus more on the cardio. All in all though, you need to have muscle to burn fat. The more muscle you have, the more fat is burned off. So, cutting strictly to a cardio workout, may not help as much as it sounds.