K.,
As far as I could find out, Singulair hasn't been studied in nursing mothers. I would be a little leary of taking it. Is it possible to take another allergy med that has been studies, such as Zyrtex or Xyzal? Those have been okayed for breastfeedng mommies. I just worry because I know Singulair is used in Asthma patients and most asthma medicines aren't okay during breastfeeding. Anyhow, it probably will decrease your milk supply. I had to take fenugreek (you can find it at most stores) caplets to keep up my supply. Plus, drink lots (I mean LOTS) of water to keep well hydrated :)
Hope this helps,
A.