Antihistamines are a good treatment for this. Studies to show it works in nasal allergies are underway. The same mediators present in asthma are also present in allergic rhinitis so it should work. Singulair has been only tested with single dose in the evening in all of its studies and that is how it has been approved for use. It would be best to check with your pulmonary specialist.
Nasal steroids are generally well tolerated during pregnancy and for nursing mothers.
Many antihistamines can also be used although their safety in nursing mothers with the newer antihistamines has not been tested. It's likely that traces of both drugs may get into breast milk but probably have no adverse effect. The exact role of all molds in causing sinus and lung disease is very complicated and they target of a lot of research, at present. Singulair was originally given at night to cover nighttime asthma. However, it is a once a day medication and as such should provide coverage for 24 hours.