This happened at the end of my stress test?

[marysmom24]

I am a 62 year old female.I had a stress test a few weeks ago and although I did get to my target heartrate, they asked me if I could go further. After a few minutes my heartrate jumped from 140 to 160 and I felt slightly dizzy.They stopped the stress test.My cardiologist wants to do a thalium stress test now.I am having a holter monitor put on this week and then the stress test.
I do take meds for HBP.I also  have anxiety and agoraphobia(although not housebound).I have a small PFO that I take aspirin for.Also migraines(complex).
My cardiogram and echo cardiogram were  normal.
My cardiologist said in his opinion there wasn't a problem, however to be safe he wants the thalium stress test done.
I was having a panic attack and very nervous when the incident happened.
Why would this occur? My doctor said panic wouldn't cause the acceleration of my heart.He also mentioned something about the sinus or electrical something or other.I was too nervous to fully listen.
Thank you.