Our baby had reflux, so we have seen our share of milk coming back at us - LOL! Our baby still throws up a lot has he has asthma, and coughs a lot, and his milk is hardly curdled when it comes back up. Many times it comes back out the same way it went in - I guess I will disagree with most of the posters above who say it always comes out curdled, as it definitely doesn't with our son. As well, we go to WDW for 2 weeks each July, and I have breastfed and given formula on days when it was 105 degrees in Orlando, and no issues, so I would agree that it might have been the milk itself, too much (1.5 sippy cups is a lot of milk for a small tummy) at one time, or baby had a virus or upset tummy to start with. The heat is hard period, so I would never give 1.5 sippy cups full of milk at one time - our DS2 gets 2 ounces at a time and never gets more than 4 ounces total. Our ped agrees that most parents just cram milk down their kids throats, and as a food, it is hard to digest, on a regular day, so with the heat, it might just have aggravated her tummy as she drank too fast, or it didn't actually digest when she laid down to rest, BTDT.
Our kids drink cold, fresh milk at WDW all of the time, as well as fresh water and juice as it keeps them hydrated, and so even if they were to throw it back up, I would never assume it was the heat at all. There could be many reasons - our ped always encourages milk even when our kids aren't feeling well, as it has a lot of water in it, so it's fine as even if they throw some of it up, they will retain some nutrients and water. My son was very ill as a baby, and all he would drink was my breastmilk - it was a Godsend as that was all he could stomach many times when he was ill with pneumonia or bronchitis.
Good luck in figuring this out, Tiger