Why do I have a damp area on my carpet? House is on a concrete slab?

There is one spot in my living room about 4x4 that always feels damp. The floors have a slight "old" smell too and I am starting to think it may be mildew. I live in a one level ranch on a concrete slab, and from what I have gathered there is no plumbing under it where its damp. Is this normal?