It sounds overwhelmingly likely that the caloric deficit that lead to the 3kg/2weeks was a bit too much.
(For reference, caloric deficit means we consume less calories than we spend per day.)
If your goal is continued weight loss, I would recommend adding some more carbohydrates to your diet. Yes, it means increasing your caloric intake, but like I said, it sounds like your low caloric intake is causing your lack of energy.
A very good way of adding good carbohydrates throughout the day, is chomping on some fruit every now and then. If it's been a couple hours since your last meal, your blood sugar might decrease, causing a feeling of low energy. Fruits are high in natural sugars, and will help combat that, and provide a boost in energy.
When we lose weight too fast, it's very likely that a good portion of that weight loss is muscle atrophy.
If, instead of losing 3kgs over two weeks, you start to lose 1kg every two weeks, that's a far more healthy rate of weight loss.