I'm wondering if some of you could explain how the leg pain was. ...

I'm wondering if some of you could explain how the leg pain was. My 20 month old has been on Singulair for 2 months for ?asthma (he's has bronchiolitis once and pneumonia 2x since he was 6 months old and a chronic cough). His medication "regime" in working great-absolutely no cough in 2 months from a child who I started to think coughing was normal for this age. 3 weeks ago we were driving in the car and he started grabbing his right leg say "ooww, boo boo". and now in the last 2 weeks we have had 3 times where he holds his right leg crying in pain and will not walk, will not stand on his leg. One time lasted for about 2+ hours-he went to bed like that and woke up fine, jumping in his crib. 2nd and 3rd time it lasted about 1 hour and then he was fine completely back to himself. We had him seen but decline xrays and to just watch it for now. The MD said it could be a virus in his knee. I asked about side effects to med and she doubted it. But I started reading and now I'm suspicious. The weird thing is that it comes on strong and goes away just as easily. Wondering if anyone else could describe their leg pain in more detail. Thanks!