Welcome to HealingWell and the Heart & Cardiovascular Disease Forum.
I am glad to know you saw a cardiologist and that all your tests turned out OK. Your Dr. has a good point about the caffeine and smoking. I do hope you will do your best to give up the smoking and cut back on the caffeine.
Anxiety and stress can also cause your heart to speed up. People who are prone to anxiety may have palpitations more frequently.
Try breathing exercises or deep relaxation (a step-by-step process of tensing and then relaxing every muscle group in your body) at the time of your heartbeat sensations. Also, consider practicing yoga on a regular basisto reduce the frequency of your palpitations.I have experienced tachycardia on occasions and once I did call my Dr. who suggested I rest and see if the rhythm returned to normal. I fell asleep on the couch and when I woke up I was just fine.
Again a warm welcome to you and keep on posting with us here in the forum.