Possibly a bacterial infection, I get them somewhat regularly due to hyperhydrosis, it's easy enough to clear up.

Trim and shave your armpits and use an anti-bacterial body wash, a medical grade one from the pharmacists. I use Bactiscrub, works great. Leave the body wash on your skin for 3 minutes, then rinse. You should be good to go, no stinky pits, at least for a few weeks. If you start to feel stinky again, just repeat the steps above.