Weird Posture Help

Hello, I apologize if I am posting this in the wrong section, but I didn’t know where else to post this. When I look in the mirror my armpits go diagonal such that when my arm is dangling I can see the bottom part of my armpit and the top is covered by my arm. I couldn’t find anything about this on the web. I’m wondering how I should go about fixing this? Any help is much appreciated.