Breastmilk varies in water content/nutrients depending on baby's needs and weather! I found when feeding mine that in hot weather I was very thirsty indeed and my milk was more watery. It's nature's way of making sure little one gets all they need.
Yup what fessmess said, if you express more later you can add it together but it needs to be used before this mornings milk is out of date iyswim? Can't for the life of me remember how long it can stay in the fridge because I freeze mine straight away
Yes you can mix any more milk you express. After a night out and few hours away from my DD I got v. watery milk, and I could feel my breasts were still quiet full. I'm not very good at expressing and find my pump doesn't "empty" them the same way DD does. The milk will be fine to use. And congrats on your nights sleep