If you are a Weak D, you are actually rh positive with a weak expression of the D, meaning that you can show up as an Rh(D) negative in some blood type tests, but Rh(D) positive in others.
A Rhesus D (RhD) red blood cell phenotype with a weak expression of the D antigen occurs in 0.2% to 1% of whites and is called weak D, formerly Du.
Source: Molecular Basis of Weak D Phenotypes
See also: 4 major reasons why blood type tests can show different results