Pimples under my breast that smell really bad when opened

Raaaaah

Go see a doctor

I'll put it a bit more delicately. It's time to see a doctor, we cannot diagnose you over the internet, we can only guess.

It could be an ingrown hair (although that would be odd under your breast).

Your doctor can run tests to determine what this is. I wouldn't wait. Anytime there's discharge with a foul smell it's time to see a doctor.

Good luck.