EkG changes but clean cath

I have had two heart attacks and have three stents. During the last two weeks I have had tiredness, unable to walk my full route and two bouts of nausea but spread out over the two weeks and no two symptoms at the same time.  On Monday I had pressure in my chest, extreme anxiety, and my blood pressure was 168/111.  I tried laying down for approximately 1 1/2 hours but the BP stayed up.  I went to Urgent Care where an EKG was run.  It showed some ischemic changes and my doctor ordered hospitalization. I was on a holter monitor and EKGs throughout the night. Blood work showed no increase in enzymes but my doctor stated that two of the EKGs showed further changes and recommended I be recathed. He contacted my cardiologist but she wanted me to come to her office rather than schedule a cath.  After her examination she arranged a cath that showed everything was OK. My question is--how could three EKGs show changes but the cath is clear.  I was not able to talk to my cardiologist after the cath. I telephoned my family doctor and they are waiting to get the report from the cath before deciding on follow up.

[Cleveland Clinic]

ECG's after previous heart attack can be misleading in terms of ongoing ischemia.  Catheterization is the gold standard test.