Why do I smell, especially when I stand up?

I don't usually notice myself smelling when I sit down, whether it be on the couch at home or at a desk in school. But when I stand up, I notice that there is a foul smell ( I don't think its body odor, and its not that smell on people who fart a lot) coming from my body. I seem to think that its coming from my rear end, but I'm not 100 percent sure.

I wear plenty of deodorant, I shower daily, I use body wash daily over all areas of my body, and I recently started wearing cologne to see if it would cancel out the smell. I've also started using talcum powder on my butt as well in the morning before school. What could it be? Thanks very much.

BTW I'm a 17 year old male, and I wash my clothes everyday. But I know this odor is coming from me