A red bum can be a few different things:
Are you wishing at home? If so, I would suggest adding another rinse cycle to your wash to ensure all soap/ detergent residue has been rinsed from the diaper. You can also check the final rinse cycle- if there are any suds- make sure to rinse again until water is suds-free.
If that's not the issue- make sure that her bum is COMPLETELY dry before putting the next diaper on. Either pat dry her after her wipedown or a little (talc-free) baby powder before the next diaper should help.
Also, when it's warmer outside, make sure to change her as often as possible.
If you need other suggestions- feel free to email me at email@example.com! Good Luck!