The answer is Yes.
While unlikely, you could have asthma, despite seemingly normal test results such as, for example, a Methacholine challenge Test. Trouble breathing can also be secondary to disease of your nose, lungs or heart. Pulmonary function testing would help determine if you or do not have asthma and, if not, might give clues to another cause of your “trouble breathing,” as might a chest X-ray.
Especially if the “trouble breathing” is of recent onset, it will be most important that you and your doctors conduct all tests necessary to establish the cause. This investigation might require your requesting a 2nd opinion with a lung specialist, (pulmonologist). You should not have to suffer “trouble breathing” indefinitely. Doctors should be able to diagnose and treat the cause.
Good luck
The short answer is "Yes."
Asthma is a disease characterized by spasms of the airways in the lungs. Patients complain of difficuly breathing, specifically exhaling. The classic finding on examination being wheezing during exhalation. This is intermittent, so your "normal" asthma test does not preclude asthma. Writing that you take breathing treatments and singulair implys you are being treated for asthma. Your runny nose may be a sign of allergic rhinitis and you may have allergic asthma. An examination and testing by an Allergy/Asthma specialist is certainly suggested, and if there are positive allergy results, specific therapy directed at addressing those allergies, may be beneficial. Asthma is a chronic disease so you would be better off getting on a regimen under the direction of a specialist to control it, rather than only using what is termed "rescue" treatments.