What in the heck happened?

I cooked a pot of chicken and noodles last night for dinner. I did nothing different that I usually do. I reheated them for dinner tonight and added some chicken broth like usual. They tasted just fine and smelled great. We all ate them for dinner. Later on in the evening I went back out to the kitchen and heard something bubbling, like boiling water. I took the lid off of the pot, (the burner was off and had been for hours) and it was bubbling! It also had this rancid smell to it. I was just wondering if anyone has any clue as to what happened. It was horrible, we carried the pot outside and dumped it into a bag and threw it in the trash. The smell is still in my kitchen. It smelled like a rotting....something!!!