Welcome to the breastfeeding club
I am currently breastfeeding my 2nd baby - Sofia and she is currently 9 weeks old. She is partly breast and partly bottle due to being underweight. So I nurse her 6 times a day and then she has bottle top up on 4 of these 6 feeds.
Like you I also lost 5 kilos straight away, then I ate too much at Christmas and have lost 3.6 kilos this month.
Finding that if I drink more water, due to extra exercise that I produce a lot more milk = needing to extract = burning more calories.
I started dieting again 6 weeks after birth and it hasn't been a problem its just a case of not being extreme.
Good luck and love your profile pic - your baby is so cute.