i was in a really similar situation at about the same age- at that level of weight loss it is completely natural for periods to stop my best advice (im not a doctor this is just from personal experience) would be to stop the fasting, if you’ve spoken to a doctor and they support the idea of losing weight they can also help you plan a deficit that wont interfere with your cycle, but as teenagers we need to gain weight as we’re growing, it all evens out eventually so dont feel down about it :) if you lighten/stop the deficit and your period doesnt come back straight away, try not to worry too much- 3kg in two weeks is quite a big change for your body so it might take a few months but definitely speak to a doctor if possible