The fact your tests results came back as normal should offer you some reassurance. Most of us get the odd ectopic heart beat here and there. Palpitations are are made worse by stress
If you try and take your mind of them and stop worrying, they will more than likely go away. The problem is, you worrying about it is making them worse and the more you worry, the more they occur. You need to break the cycle. Everything is fine, you can relax.