Does anybody know ?

What this might be.
Took Mum for her 3 monthly check up yesterday and apparently for her last two blood tests it has shown that her red cells are changing (getting smaller) and losing their
colour. Dr is now having her tested for something in particular didn't say specifically what but mentioned she may be low on iron but not anaemic as her hemoglobin count is spot on.
Was wondering if anybody had any ideas what it may be.