Half of it was due to my stomach not liking gluten (or something similar). I've had three periods where I haven't been able to eat properly becasue of it.
The rest is just because I have significantly increased the training and haven't worked out how to get enough food inside me. My stomach feels permanently full so I can't eat any more.
I'm not worried about it yet. I still have at least another stone of fat on me so I'm not about to starve. I lost weight in a similar manner when I started marathon training, until I learnt how to get the balance right. If I get down to 70kg then I will start worrying. I've still got 7kg to go before I get that far.