I was under the impression that inhaled lead (paint dust from sanding) is
absorbed at about 90% efficiency whereas ingested lead is WAY less effective.

A few weeks to get elevated levels seems pretty quick. I'd look for other
sources of contamination & I'd get the blood tests redone.

Seems like the bath tub (paint's on the outside? porcelein on the inside?)
would not be the source.......

keep us posted