My heart freaked out while I was exercising?

I stopped exercising and checked my pulse and my heart rate was really fast and I was having heart palpitations up the wazoo! I've been to a cardiologist. I went in January and they did an ekg, holter ekg, stress test and the chest ultrasound and said my heart was normal with benign palpitations. But today made me extremely nervous. I am afraid to sleep. I feel fine right now just extremely tired. And after I exercised I felt out of breath and thirsty but that was about it. I just tried to get my heart rate down cause I was afraid I was going to go into cardiac arrest or something. I haven't exercised in a over a month so idk if that's the reason why but I'm pretty scared right now. Help please