I used to have a period of time when my heart rate was 120-145b/min and high bp, and now it is below 60,and both at rest. what's the cause of changes?

Atrial fib to brady: Atrial fibrillation turned into bradycardia. Sometimes, in the absence of treatment, which I assume to be so in your case, atrial fib spontaneously converts. I appears that yours has spontaneously converted into a state of bradycardia (low heart rate); this is problematic as you may not be getting sufficient blood to your brain and other organs. You need a consultation with a cardiologist