Wow Sparkler, this is almost the same situation as my DD (now 10) has been having. She became friends with a group of 4 girls in Reception. They were always close until about yr 3 when cracks began to appear in their relationship. Things got slightly worse during year 4, and like you say, there was a lot of friends one minute and not the next etc. During year 5, the situation got a bit out of hand, my DD had gained a bit of puppy fat, and these girls as well as ignoring her, whispering about her in a little huddle whilst looking over at her, began to call her fat. They said to one of DD's other friend's (who wasn't involved in the bullying) "you've got a fat friend." And horrible things like that. Only one of the Mothers did I know fairly well, but I wasn't friends with her exactly. The other Mothers I rarely saw, so I dealt with it by going into school and speaking to the class teacher. I had to do this twice, and eventually the parents were brought in. The Mum that I know has never spoken to me again.
DD, now in year 6, has had a few other 'fat' related comments, from boys this time, more recently, and the effect of all this happening over time is that she has been suffering from depression and anxiety, to the extent that she's had thoughts of self harm. We have seen our GP and she has been referred to a Counsellor.
This IS bullying, and is the kind that girls specialise in, verbal nasty comments that eat away at the victims self esteem. I hope your DD's problems at school soon end, and that she doesn't suffer to the extent that my poor girl has. She's had trouble sleeping, tummy aches in the morning (anxiety) feelings of self loathing and uselessness. It is hard to take it up with the parents, but deal with it in the way you feel best, whatever that may be, do get it dealt with, as what starts out as seemingly harmless enough girls falling out, can escalate into bullying issues. Good luck.