I've been on Montelukast Sodium 10mg for the past year for allergies & asthma. Whenever I skip a dose, my stomach gurgles & gets an empty feeling. Why?

Interesting. I have never heard of this effect before but can conjecture that leukotrienes may be causing your stomach symptoms . Since Singulair (montelukast) blocks leukotriene action, omitting it allow the leukotrienes to "attack" your stomach. But this is just a guess.