What Causes Bad Odor From Armpits?

I have noticed really bad BO under one armpit the last 3 weeks. Some days its not so bad, but other days I smell like a man. I have really good hygiene and have worn a clinical antipersperant/deoderant for many years. I put the antipersperant on before bed and wear a deoderant after my shower in the morning. And Its only my Right Armpit.