Sudden Anxiety in 6 year old

I would suggest monitoring her diet as it could be linked to an intolerance or allergy. She has been tested for milk / gluten allergies right?

My son was exactly the same last year in Primary 1. He was the most confident child in nursery and even the first few weeks in P1. Hi anxiety started about this time of year and I have no idea what brought it on.
His fear of getting a 'Red Card' made him cry every morning although he'd never been in trouble before so no idea why he was so afraid of it. Also, on a Monday they did 'weekend news' and every Sunday was spent in tears worrying about what he would write/draw. It was awful! He also cried going in everyday.
It lasted the whole of P1 and he struggled slight in the first few weeks of P2 but glad to say he now goes in fine.
He didn't have the same anxiety with food as such, just school in general,
Fingers crossed it's just a phase and it will pass for your DD too.

This sounds very worrying OP, and I'm not surprised that you're concerned.

Does all seem ok at school? Any friendship issues or anything like that?

For the moment I would carry on as you are. Lots of reassurance and cuddles for DD, stay calm and hope things get better. If there's no improvement by the end of term it may be worth seeking professional help.

has she mentioned anything about friends? Are things ok there?
Could there be a child who's being mean to her, or anything like that?
I think at this age, a lot of children find it difficult to articulate what's wrong. You may already do this, but do you give her plenty of space to open up? sometimes it's good to chat away about yourself, rather than asking any direct questions. for example, "I remember when I was at school, there was a time when my best friend stopped playing with me and I felt lonely. I had no one to play with and it was sad..." Or "I remember when there were a couple of kids who kept saying mean things to me.." She seems to be targeting food but that could be just because it's something she has control over?
Another helpful thing is to make sure she knows that she is secure in her relationship with you by having 'special time' with her every day (even just 10 min focused time), words of affirmation, whatever her love language is. it also helps to know somebody unconditionally loves you and to really experience that.
If you're playing with her, you could suggest you play school with different dolls etc, and see what comes out of that. Maybe she'll act out some of the stuff she's worried about with regards to school? Not all kids are going to do that, but I remember when my dd was little, she'd often act out in her pretend games things that were going on.
I don't know how helpful that is, but I really feel for you, it's really hard when your child is going through something and you don't know what's causing it or how to help. i've experienced that myself and there are no clear obvious solutions. I'm glad the school is being supportive and wish you all the best with this!

P1 is also quite a big step up from Reception and it could just be that she is finding it hard to adjust and quite overwhelming. It's really reassuring to read the other posters' comments about how their children went through a hard time and came out the other side.

My daughter projected her anxieties onto food around the same age. No real reason for the anxieties (and have since learned that it's not necessary to know why, my daughter is just an anxious individual) . It's about teaching them better coping strategies to deal with their anxieties, and helping them to talk through their fears with you. In my daughters case it came and went for many years but started to take a bigger hold around 10 or 11. At 12 she was diagnosed with anorexia and we have battled ever since. That no way means this will happen to your daughter at all, but just think it would be wise to help her to develop other ways of dealing with her anxieties and may be seek some professional help. But its also worth remembering that lots of young children experience anxieties as they grow and experience changes in life like new classes/schools etc. Hope it improves for you OP.