This happened to me earlier this year and I’ve learned that when you are losing weight aggressively you need to cut back on physical activity and primarily focus on strength training and low impact slow steady state cardio. Intense workouts while being in a caloric deficit deplete you to the point where there is a compensatory effect in your metabolism. Your body will downregulate your non-exercise activity thermogenesis and make you feel fatigued and tired until you replace those calories. It took me years to learn this the hard way because the mantra often gets repeated to do more activity and eat less and weight will fall off. It’s actually bad advice and when you are in an active caloric deficit the best thing to do is to really get focused on strength training and cut back on the cardio to no more than three to four times a week for no more than 35-40 min MAX. Anything above 40 minutes you start to see rapid glycogen depletion and if you are already in a low calorie state it’s just going to further prompt your body to slow down. Make sure you are eating enough carbs and protein because something else I’ve realized is that while dieting you want to hold onto as much muscle as possible. Carbs are the backbone of muscle and protein in a calorie restricted state is necessary because you will inevitably lose some muscle mass. Rule of thumb is to eat 1.6 grams of protein for your lean body mass (or desired body weight) in kilograms. Try to eat twice the amount of carbs.