Similar to all the other posts, breastfeeding really took it our of me.
I think I noticed as I BF for so much longer this time (12months)
I am down to the lowest I have been since before having any kids!!
One thing I did notice, once DD2's feeds started to space out and she was feeding less, things seemed to stabilise for me. I was no longer STARVING all the time and my weight seemed to settle down. I think (as I cant see your ticker atm) that your little one is still fairly young?
Things may even out but probably best to get checked out just in case -4kg in 2 weeks does sound like a lot.
Mr J (April 2005) Miss Z (Feb 2007) and Miss O (Oct 2010)