It’s probably the amount you are eating vs amount you are working out. Under my weight loss FAQ section I have a formula to calculate the amount of calories you should be eating based on your lifestyle. You may want to give or take 100 calories depending on results (or if you feel weak please do eat more!) but that’s a good jump off point.
I would start counting calories based on that. It’s not a good idea to increase the intensity of your exercise to make up for how much you are eating because that can be a slippery slope to over eating or over exercising.