Re your query about anal odour - has your doctor examined your anal area?
You could have a fistulae - this definitely causes an odour.
You might have small abscess?
You could also have a cut or tear inside your anus due to straining on the toilet and this has become full of bacteria and/or candida?
On another post someone advised that they constantly felt sweat on their buttocks, and at times their backside was uncomfortable, when they tried Nizoral cream inside their anus and took grapefruit seed extract the odour went away. They believe they had a candida infection.
You would be better getting your doctor to have a look - perhaps he can give you an anti-fungal pessary to try - you can Diflucan over the counter, or an antibiotic, or an antibacterial cream.
You obviously already have problems with your BMs if you have IBS, perhaps it is some sort of impacted hard fecal matter - have you ever had colon hydrotherapy?
Although the smell is seemingly coming from your anal area perhaps the problem is deeper rooted in your bowel - in the first instance it would be better to speak to your doctor and see if he/she can trace the problem.