the rash might be a simple case of eczema. I would make sure the dr you see or saw is a dermatologist. If it's eczema, a topical steroid will do the trick to resolve. But it sounds like you are definitely clear of syphilis, especially having gotten the penicillin shot.
Rash could be so many things, would have to see a DR.
What else could cause a rash on my palm and foot? It seems so conincental
No, that time frame was more then fine to detect a syphilis infection. Also you were treated so even if on some very very very rare chance it was a false negative it would have cured the syphilis.