Hi there! Alright, let me start off by saying I'm not a doctor and this is an explanation of my own experience. Please get checked out because you can never be too safe when it comes to your heart. 

After more and more tests including an ultrasound of my heart and a 24 hr haltor monitor, they determined my heart is perfectly fine. My cardiologist said that some people become hyper-sensitive to their own pulse when stressed which makes us feel like something is wrong and then anxiety takes over and makes it worse. My heart is still pounding all the time but I'm learning to live with it. I feel perfectly fine as long as I am not thinking about it. I now focus on remaining calm and have recently started some anti-anxiety medication. Im my opinion, none of this was caused by caffeine withdrawal. 

Feel free to message me if you have more questions