The dr's urine test may have been weaker than the one you used at home, hence the neg, everyone has an amount of hcg in the bodies anyway, so I guess it depends on what level they classify as POS. I know most of them classify over 20hcg as pos, put because we all increase our hcg rate at different levels yours may have been lower. It is definetly possible. After my month I think anything is possible. Because your rate should usually double every few days they will just check it again in a few days to make sure that whatever level it was at it has increased. It is very frustrating.
Good luck and baby wishes.(If thats what you are after) Hey at least it will make up to an even number. LOL.