Hello and welcome To HCM forum .
It is very difficult to pin point simple increase in monocytes to any particular disease without a detailed clinical examination .
Headache and nausea can be because of anemia
, infections and many other factors .Monocytosis
or increase monocytes in the blood can be due to stress, viral fever
, chronic bacterial or protozoal infections , malignancies etc Most of the infection swill show a blood picture of increased Total Leucocyte counts .
Some children can me more sensitive to common moulds (found in basements ,cushions ,mattresses ,damp walls ,gardens etc )and may exhibit cough , breathlessness fever etc as the symptoms .
I suggest a comprehensive allergy
sensitivity testing which helps in finding out the relative sensitivity of a person to common moulds, foods etc .
Hope this helps ,take care .