Should I be concerned ?

I'm 17 weeks pregnant and haven't put a lb on, in fact I've lost 8 ! jeans that were too tight pre pregnancy fit me now so it's definitely come off my legs and thighs and bottom , I have been walking more than pre pregnancy though as my daughters nursery hours changed so I do have to walk there and back 3 times a day instead of 2 , it's still coming off I'm not complaining but being 3 weeks off half way I'd of thought I'd of gained not lost ? and my bump is huge too being my 4th and baby measures on the large end of the scale for my weeks .... I'm eating alot too I'm always hungry ! is it ok ?