Hi as i am a newbie to this forum please forgive any mistakes i make

I recently bought a 2004 C200 deisel manual estate and i have noticed a very pungent fishy type smell a couple of times.

Both times it was whilst in traffic on a hill could it be the clutch?
It is a really bad odour that lasts for days
I thought at first it might be fumes from car in front or even A.C but it happened again yesterday and the gears got stuck in second --

Can anyone help me please

Fluke