Singulair and observation

I am 47-years-old and have been on Singulair and Zyrtec daily for at least 10 years. I have to say I have never had a single side effect or episode of depression from these drugs. I have a condition known as idiopathic anaphylaxis, and these drugs have been literally a "life saver" for me.

I have been taking Singulair since the late 90's and it was a real blessing, it seemed, as far as my asthma. I have not really wheezed since. However, I have been gaining weight at the rate of 1 pound per month steadily in spite of eating reasonably and exercising 4-5 times a week. Additionally, I am unable to stick it out through an entire aerobic routine and have been working for over a year to get better -- no amount of effort seems to work. So, in spite of not wheezing, I'm still no aerobically fit. I had a pulmonary-cardiac test recently and the Dr decided that I really do have asthma problems even though he could not detect them by observation and my answers to question otherwise.

So, I'm wondering if Singulair is a culprit in my weight gain (seems to be) and inability to shed it. And I'm also wondering if Singulair isn't really helping the asthma but just hiding the symptoms.