Poor you, and for your DD.
FWIW, I canremember being 9/10 years old in Year 5 ... I wasn't unpopular, but deinitely went through phases of being left out and it was incredibly painful :( I was tall, maybe a litle plump, frizzy hair - and probably one of the kinder kids in my class. Young girls can be so vile, especially when they join together at the expense of another - usually quite sensitive - girl.
I got through it, as lonely as it was, but I won't lie and say that it was easy. I found it difficult being in the playground on my own. It made my little heart hurt. But, girls are also fickle and I was able to join in with another sympathetic group when it became obvious to them that I really had no mates at all.
The problem was that I gave too much of a shit. Try to encourage your DD to care less and she'll probably find that friends will flock. That is such a difficult, abstract concept for a little girl to grasp but if I could go back to 1993 and give Little Me a piece of advice, that would be it.
The list that another poster gave earlier is excellent. Make sure that your DD is occupied and that teachers are aware of the situation.
And do make sure that you give her lots of extra cuddles, and do nice things together out of school time. You seem like a lovely Mum, so I'm sure you're onto that already.
And, as for me, I'm 29 now, I have a wonderful husband and two small girls (3yo and 10wo). I have a circle of very good friends and those days are a distant memory. It will get better for your LO as long as you are there to look out for her x