This was me. I was tiny until I got to 15 and my Mum became really ill and was hospitalised for a year. My Dad worked nights but would cook an evening meal.
With no supervision, we would go down to the shops and buy 2l tubs of ice cream and sharing sized packs of Malteasers and the like, every evening - and eat the entire lot. I got a part time job in a takeaway, which just added to the problem.
My sister and I both ballooned. I went from under 8 stone to nearly 11 in little over a year. I was so miserable. It took me till me mid thirties to get back to a healthy weight; I'm now 9 stone - still only 5' 3" though!
I know lots of PPs are saying NOT to address it with your daughter OP, but I WISH I had had my Mum to speak to about what was happening. And to help me. She was in no fit state to do anything and has carried the guilt of 'abandoning' her girls to years of being miserable about their bodies.
I have 11 and 9 year old DDs, and if one of them started to put on a lot of weight, I would absolutely address it with them - I wouldn't want them to go through years of being miserable about their size - it simply dominates EVERYTHING when you're big (and I was 19 stone at my heaviest).
Good luck OP, I hope you can find out if there is something underlying and help your DD.