To bsheahan…
It certainly sounds like paroxysmal SVT which comes on when one has an “extra” electrical circuit/pathway (often born with) and either a PAC or PVC happens at the “right”moment of your heart's electrical cycle which causes a sort of short circuit, and thus the sudden increased heart rate. Mine manifested when I was 34 and I’m now 66.

Positional changes such as bending over to pick something up, commonly will trigger a PAC/PVC in me, and thus an episode of PSVT. Other triggers can be eating/swallowing etc. Stress can increase the number of PACs and PVCs, and thus increase the episodes of PSVT, but if you didn’t have the abnormal circuit in your heart, you wouldn’t have the episodes. The hallmark of PSVT is sudden onset, and just as quick, termination. Panic attacks are more gradual in onset and termination back to a normal heart rate.
You may need to be on a Holter monitor for weeks to months to capture an episode and get an accurate diagnosis.
I’ll offer another trick that has worked wonders for me over the past 3 months whenever I have an episode. I used to use Val Salva techniques which worked for the most part, but now all I do is sit with legs apart, and drop my head between my legs for 5-10 seconds and breathe slowly. It gets my heart rate back to normal every time. (Fingers crossed). I hope this helps.