anythingnoteverything years ago, HVs used to talk about foremilk (watery) and hindmilk (fattier). It was a very simple way of describing how the composition of breastmilk changes throughout the course of a feed.
When I trained as breastfeeding supporter a few years ago, we used to talk about the "three parts of a feed":
1: this first bit used to be described as "fore-milk", just after let-down, quite watery, this is the SOUP STARTER - quite watery (sometimes thirsty babies just need a drink and stop after this bit).
2: the second bit is when babies settle down to serious feeding (suck-suck-wait, suck-suck-wait, with lovely fat little cheeks as they suck). This is the MAIN COURSE, and this part of the feed can go on for ages depending on how hard the baby sucks. The fat and nutrient content is much higher than in the first part of the feed. This is "hind-milk".
3: the third bit is when babies look like they have finished, but haven't quite let go. This part of the feed is sometimes described as "just messing about at the breast" or "using you as a dummy". The babies might look nearly asleep but, if you poke them or twiddle their ears, they latch back on and do fluttery sucks that are highly efficient - this is the very valuable CHOCOLATE PUDDING part of the feed, very high in fat and nutrients and should not be missed by taking the baby off the breast too early.
Now we don't even talk about 3 parts of a feed, because it is still too clear cut and formal. The composition of breastmilk changes throughout a feed and to suit your baby, depending on the way your baby sucks.
Frogs - thinking about it, too much "fore-milk" would not really explain the back arching etc. Obviously you should try compensating with making sure she gets enough "hind-milk", but be prepared to go to the GP if necessary.
If it is a reaction to dairy or any other food you are eating, please be reassured that most babies grow out of this. My DS1 is one of the ones that didn't, and I have found much support on the allergy boards.