G.B.
She's just growing and her body has different needs. When she's older you won't see the phases as readily. Kids who are having a growth spurt will start eating everything they can get their hands on, they eat and eat and eat. Then they chub out and you're thinking they are just getting fat. But they're probably not. They will chub out then shoot up one night while they're sleeping. Then their clothes will fall off them and they aren't eating much at all.
In a small baby their phases aren't as obvious. She's eating a bunch because she's about to grow a bunch. Then she'll go back to a normal sleep cycle and crying it out won't be what caused it.
Letting a baby fuss for a few moments isn't a harmful or cruel thing but when a baby is all and all out crying they don't understand why mom or dad aren't coming to see what they want. They are learning to trust or not trust those who take care of them at this age and letting them lay in bed and cry for extended periods of time is just cruel. I know that isn't what you mean when you say CIO but please do know she's learning to trust you and count on you. If she needs or wants you there are plenty of years ahead of you where you can say no to her.