Hi Gen Jatinder,
I apologize for delay in replying. Get your device examined by your cardiologist who will call the technician of ICD you have been given. If there is any arrhythmia
that will be picked or if there is device malfunction that would also be diagnosed. To me it appears unrelated to device it could be postural hypo tension you are taking B Blockers or it could be some vasovagal phenomenon. Keep yourself well hydrated in summer. Last but not the least: consult your cardiologist would be my advice General.
Dr Anil Grover