I stopped taking it after 3 days without tapering and I feel visi...

I think you should tell your doctor what is going on. The research I have done concerning Prednisone tells me this:
When you are taking Prednisone your adrenal glands get LAZY, waiting for the Prednisone to do the job the glands do normally. When you are no longer taking it or being tapered this causes a fight or flight situation causing the body to over compensate. This action sends more adrenaline than is needed causing heart rate to increase with possible cardiac trauma.
Hope this information helps.