What Causes Periodic Shortness Of Breath?

I was prescribed the 5mg tablet of oxybutynin for OAB by my gynecologist on 4-1-14...After taking the dosage for approximately 3-4 weeks (and experiencing episodes of shortness of breath), I landed in the ER for observation of this shortness of breath...All tests performed (i.e.EKG, echogram, CT scan of my lungs, chest x-rays, numerous blood draws) were negative (as to lung capacity, heart enlargement, etc.)...My follow-up visit with my gynecologist today (6-23-14) was basically a Q and A, with the suggestion made that I could try Enablex (too costly) or try oxybutynin again for a trial...I am somewhat hesitant, fearing a repeat performance of my ER visit...My MD seemed to feel that the shortness of breath was an anomaly not directly related to the oxybutynin...Thanks in advance for any insight you may have...