Hi there ~
I understand that your panic attacks are causing you a lot of distress, to the point where you are thinking about when the next one is going to happen. This is called anticipatory anxiety
and may lead to precipitating another panic attack. I think that with all the anxiety that you have been having, having variations in your pulse rate is not uncommon. I recommend that you get an electrocardiogram done, and if there is anything suspicious a cardiology referral is in order. I hope you are using relaxation strategies to control your panic attacks. You should see a psychiatrist
to have your panic attacks treated with medications and get appropriate therapy referral.
I hope this helped. Take care.