Rapid heartrate is one of the foremost panic/anxiety symptoms. Distraction works great for me too. I agree that if it goes away that easily, plus you've had some tests done, that it is the A/P.
I have a newsletter from a local hospital I received many years ago that I refer to every now and then (less often since I found this site!). I use it to reaffirm the cause of my symptoms. Here are a couple parts of the "article":
"Suddenly, you are gripped with intense fear for no reason at all. Your heart races, you palms sweat, your chest hurts. You think your're having a heart attack. You think you're dying. You're having a panic attack."
"According to the APA, a panic attack is an unprovoked surge of fear accompanied by at least four of the following symptoms: shortness of breath, a smothering sensation, dizziness or faintness, accelerated heart rate, trembling or shaking, sweating, choking, nausea or abdominal distress, feelings of detachment or unreality, chest discomfort or pain, numbness or tingling, hot flashes or chills, fear of dying, fear of going grazy or losing control."
While I usually experience the faintmess, racing heart, hot flashes and detachment, I have had all of the above symptoms in various combinations!
Re-reading the article helps remind my brain (which seems to take control during an attack) that I am having a panic attack, which then makes the relaxation, breathing, etc. more effective. It may sound silly cause we all know too well the symptoms, but seeing it in black and white during an attack or bad period (especially when it has been a long time since the last one) helps me acknowledge and get a handle on it.