Do you think you had a propensity for panic and the singulair just brought it out? Would think this could happen. Singulair does tend to be linked to psychiatric issues and panic especially in pediatric populations (up to 19 yrs). Unfortunately, now it would be best to avoid all medications similar of that class in the future. Unfortunate as being asthmatic can definitely make life also quite difficult. I feel for you. Have you ever tried acupuncture? What does your doctor say about this situation?
I've never taken it but a lot of medications, a whole lot, can cause psychological problems. Usually these will stop when either someone gets used to the drug or stops taking it. But here's the thing with psychological reactions to things -- in some people even when the trigger goes away the brain holds onto the problem, in your case, anxiety. Can't say for sure that's what happened, as I don't understand the intricacies of this particular med, but it's probably what is happening, as the drug isn't in your system anymore and therefore its presence isn't causing the problem anymore, your brain is. Just knowing this by a person not really prone to anxiety might be enough for you to relax and let it go, but it might not. The best thing to try first since it has been 3 months is what anyone with a new anxiety problem should do, which is see a psychologist who specializes in anxiety treatment, probably someone who knows techniques such as CBT or hypnosis and go about learning how not to let your brain expect to have panic attacks. The only other thing to do is wait it out under the assumption the drug profoundly messed with your brain and it is just taking some time to go away, but the longer you wait and the problem continues the more you will become accustomed to thinking this way and develop a chronic anxiety problem. I found out the really hard way, when a drug messes you up, nobody really knows why it happened or even what happened. Drugs are approved knowing they are dangerous, and are kept on the market knowing more and more problems arise with them on the assumption that it's better to have the treatment than not and better to have a vibrant market in selling medical services including drugs than not to have one. That means there is no reward in our system for doctors having to know how to deal with these adverse consequences, and if the manufacturer knows they aren't talking because it's bad for business. What you do is up to you, but again, the longer it goes on without resolution the harder it will be to deal with if it sticks around. Peace.