I never had problems with my period and now that I am pre-menapausal, I noticed a foul odor. I want to know why does it smell so detestable.

You could have another unrelated condition, such as a bacterial infection (vaginosis) and/or candida. It might just be 'old blood' that has been exposed to air for a while, but we aren't there to see or smell it, so you might as well go get a swab and culture.