Do you notice the difference between water and milk and just water during training?
The headaches could also be blood sugar related.
Could be a bit of both, with a nervous stomach/headache thrown in! My feeling is that having the milk gets her to take in some more fluids and have a bit of a blood sugar boost as well, even though she always takes a snack. She tends to finish one 800ml water bottle, refill and drink a little more and have 200-400ml of chocolate milk (depends on the day as to whether she finishes it). She tends to get stomach aches if she eats too much before practise so blood sugar could easily be a factor. Could also be a placebo effect - who knows? She is really hard on herself and is not a naturally 'brave' kid, but tries really hard to please her teachers (first born!). I believe my DD worries herself into nervous stomach/head aches on occasions when she perceives she is under pressure and not getting skills as fast as others. I have had a chat with coach and he now reframes things a little for my DD (and the other younger ones) which could be making a difference for DD too. He has a tough job with varied ages 7 - 18 YOs in her L4-8 training group but has done a great job listening and dealing with my gymnast as an individual so far.