Young kittens (<4 months) are very susceptibe to FeLV infection.  However,
three days elapsed between  her initial exposure and testing?  Effective
transmission of FeLV requires *prolonged*, intimate contact or a bite wound.
I usually rely on ELISA negs, however, for your own peace of mind and to be
extra sure, you could have her retested in a few days to a week.

False ELISAs are extremly rare unless there's been a human error.  The only
other possibilty for a false ELISA negative is a latent infection.  In
latency, the virus doesn't reproduce and thus doesn't release any P-27
antigen that can be detected.  Latent infections are also rare.

The in-house FeLV ELISA Snap Test (enzyme-linked immunosorbent assay) is
extremely sensitive - about 100 times more sensitive than the FeLV IFA
(immunofluorescent antibody).  The ELISA dectects antigen *circulating* in
the blood long before the infection reaches the bone marrow - which can take
as long as 1-3 months.

If there's any P-27 circulating in her bloodstream, the ELISA should dectect
it within a few days.  However, a single ELISA does *not* mean the cat is
infected or harboring an active infection.  If she does retest postive on
the ELISA, have her retested with IFA in a month.  Most cats that test
ELISA+ for FeLV clear the virus.  So *please * don't make any life or death
decisions based on a positive ELISA/

Good luck.

Phil