It happened to me once, it's from the wet laundry I left in the washing machine for two days...
Even though the laundry was properly washed, rinsed and EVENTUALLY dried, it still generates a really nasty smell AFTER it's dried...
My father actually told me my room smells like shit literally..
I only got rid of the smell after I used some really powerful laundry booster.
Now, I would never leave my wet laundry for more than a day again.