Why are these folds happening?

This is my first time seaming sleeves into a top and I seem to be getting these weird folds right above my armpit that get worse the more I move. I’m wondering what is causing this. I was thinking that I maybe messed up the length of my armhole, but honestly that’s just a random guess :/

Pattern is Cecile from Jack Frost Volume 27. I’m using a mixture of hand and machine knitting.