All my 10 ball pythons have lifted themselves during eating to help get the prey down, multiple times. However there are a select few that always do so it's normal and nothing to worry about
As for sexing, they can be sexed at any age. Breeders will sex their hatchings using the 'popping' method. Older snakes up to adulthood: you use the probing technique. Check on youtube for vids on how to do it. Ralph Davis has a few good vids on popping (
HERE)
I'd recommend probing over popping if you are inexperienced or getting someone else to do it for you, such as a vet or a breeder that knows what they are doing.