Hello,
There are many things that could cause that from an arrhythmia to a panic attack -- both can feel the same. You should see a doctor and talk to them about your symptoms. The fact that it slowly decreased suggests a sinus tachycardia, rarely an atrial tachycardia. Most other variety of SVT (supraventricular tachycardia) have an abrupt onset and termination. Events like this can be triggered by stress, sleep deprivation, hormonal changes, caffeine, or energy drinks--although sometimes they happen out of the blue.
The challenge is that if this the first time it happened, there is no way to know when and if it will every happen again. This makes catching it on a monitor difficult. It might take some time to figure out what the cause is. The most important thing for now is to talk to a doctor and have some initial tests done like an EKG, lab work, history and physical and possible an echocardiogram.
I hope this helps. Thanks for posting.