With such a topic so broad we truly try to cover the basics from all angles in this group. Nothing too big or too small. Nutrition is as significant if not more as exercise is to reaching your goals so learn all you can.
If your bf% hasn't increased or has decreased, then I would think that it is muscle. When I had the means, I would check my bf% (while trying to lose weight) and found if my bf% decreased, and weight showed a loss, then I was losing fat not muscle, but if my bf% increased along with my weight it was a good possibilty that I had gained more fat.