Actually iwas looking this one up not long ago and i think i have your answer.
No matter how careful we are with eating loosing weight does mean constantly taking in less calories then our body needs to remain at a stable weight... obvious enough
'cause we don't want our stable weight to be what it is now.
Unfortunatley thats not the way our body looks at it. As far as it's concerned we're experiencing a mild famine... it doesn't realise that this is elf imposed. Now we can do all the tricks to stop our body going into starvation mode and hanging onto fat ike grim death (like excercising to raise our metabolic rate, getting a good amount of sleep and eating a good full diet which MOSTLY covers our bodies energy needs) but our stubborn little animal brains say... whoa there: we're getting less food then what we need meaning we HAVE to burn fat to survive... not a good enviroment to get pregnant in. Therefore the body puts all the reproductive cycles on hold til times even out.
Funnily enough it doesn't happen to everyone and when it does it's chaotic... a missed period or a late one here and there, sometimes a gap of several months. But basically it's got to do with the whole reduced calorie intact/fat burning thing. Make sure you're still getting a good dietry intake and are never letting yourself go hungry.
Just in case I'd get a check out from a doctor to rule out anything else that may not be related to weight loss but I believe that most of the time this is the cause.