Armpit pain that only comes when you're tired

When you start to feel body aches or are tired, pain can occur in the armpit.

Since I have experience with breast cancer surgery, I become sensitive whenever I feel pain in my armpit.

I have been to the emergency room because of severe pain, but they say there is nothing wrong.

Today, I pushed myself a bit again, and as expected, I experienced armpit pain, so I took painkillers and used a massage gun to massage.

I can't even ask someone to massage my armpit. ㅠㅠ

Why does having fatigue cause armpit pain?

Are there anyone who has had a similar experience to mine?