What's happening at school? If she's one of the first to hit puberty then she may be receiving some unwelcome attention/nasty comments. Hormones can be hell on earth, especially in the months before periods kick in. Right now it sounds like she's probably feeling very insecure and vulnerable, hence she wants her mum. That insecurity is probably not helped by her being aware of you having financial difficulties.
Can you sit down and talk to her without distractions? Ask her about school in detail, how she's feeling about the way her body is changing, what else is stressing her out? It might even be worth talking to her teacher to see if s/he has noticed anything (especially if your DD is receiving comments etc). Another possibility is suggesting your work together at a task (she peels the spuds whilst you prep the veg or something) so you're together but she's focused on 'something' as opposed to her mind running wild.
The wanting to disappear comment is concerning, depending how your chat goes it might be worth talking to your GP for some advice as well.
She is 10.5 exactly, started getting breasts and spots despite being only 5 stone weight (I thought puberty started at 7 stone ) and very active. That's one cross to bear for her but her mood swings dictate the mood of the house. She comes in from school and is sullen and on the verge of tears all the time. Today I took her to buy a treat from the bakery and she couldn't decide what to have so she cried! Then someone said something (not mean) to her and she cried, then she came home and bawled for half an hour. She says she doesn't want to be like this and doesn't want to be a moody teenager but that's exactly how she's acting. She has always been the sweetest girl but now she is angry a lot of the time. We are struggling financially and she's aware to some extent of that but otherwise we have a harmonious family life. I do my best but she wants me to listen to her read out everything she wrote at school today in her too-quiet voice and I have to get dinner ready, hang out the washing, etc so I can't. I want to do more for her - but she wants me to herself all the time - half an hour is not enough. She says she wants to disappear altogether which worries me a bit. She's not happy! What can I do????