If you can't see anything, then you didn't contract a parasite or anything like that. I'm not sure what you have, but if your doctor and chiropodist can't see anything either, then I advise you to treat the itch with a topical anesthetic. My favorites are Sarna and pramoxine, both available without prescription. Apply them liberally and often every time you itch. You should be able to break the itch-scratch cycle that way.
If that doesn't clear you up in 7-10 days, I'd suggest seeing your doctor again.