Hi, sorry for not getting back until now. You're experiencing a sinus tachycardia. So, your heart rate is speeding up, but it's still a normal heart rhythm. Reducing or cutting out coffee can help. It's generally thought of as a benign vasovagal event, meaning that it just happens with no 'disease' process underlying. Last time it happened to me, I leaped onto a play structure with my kids, and instantly went into a sinus tach. BOOM BOOM BOOM BOOM. It's quite uncomfortable. Just rest and the heart rate should drop back to normal. If not, or if it's continuing to bother you, visit your doctor. A holter monitor will pick up any irregularities. Having a doctor actually hearing the cardiac event is a slim chance.
I've never had a problem with palpitations etc interfering with freediving. Hope you don't either
always open to more questions/comments...