What Causes Sudden Weight Loss After Child Birth And Fatigue?

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Hi. Iv been loosing weight ever since i had my son eight months ago. Iv been to my g.p loads of times about this now dey have done loads of blood test for everything and have not found anything wrong.now since the Xmas d weight is falling of me i now can feel the lightness of my body. Im constantly wore out phisilly drained and takes me so long to do things id normal have done is less den half d time it takes me now. I eat very well everyday so tats bot a concern d g.p even gave me ensures to trry put on weight and didn't work. Wat could b d cause of this. My normal weight is nine stone d last time i was weighted i was seven stone now i fear of been weighted to b told in under weight.