During a growth spurt, how do i know if my LO is crying because she is hungry or tired?

I am following 3 hr EASY, and in the last while, she would only cry if she had bad gas or she was tired. LO just turned 12 weeks, and she has been impossible to put down to sleep in the late afternoon and bedtime (screaming/crying like you wouldnt believe). She will nap well in the morning, and horribly in the afternoon.

Is she going through a growth spurt at 12 weeks/3 months? I have no idea whether she is hungry or tired anymore. Im scared that i will overfeed her because yesterday she spit up so much that i had to change her outfit twice after each feed.

I am just so frustrated since she had JUST started sleeping through the night. I feel like i am going backwards.