Also have a look at the next opportunity - there are two black 'dots' either side of the necessary exit. These are actually glands that have a secretion to 'aid the passage of solids' and can get occasionaly blocked.
Either get the vet to check them, or if you have a few nose pegs, very very gently apply a little pressure to them holding some tissue over them first, and you may get to clear any blockage that can cause smells. The vet will be the best bet to be honest as they will also be able to confirm that all else is ok at the same time.