I've had exactly the same problem with my kitty, but not all the time. She seems to go through phases. I remember reading a previous post about something similar, and some members said it was something to do with the anal glands (the two small holes either side of the bottom), if I recall correctly.
I believe it can be helped by pressing on the area with a tissue, but I've never come across a cat that would be passive enough to allow that!
I've not taken mine to the vet, as it comes and goes and she doesn't seem affected by it in the slightest. It just offends my nose! But if in doubt, then by all means ask the vet just to be sure, especially if it's a permanent problem.