I'm 19. I just recently was at work and stressed as hell. Then I had the THUD in my chest. It scared me shitless. I figured it is a number of things:
1: Acid Reflux; this can cause mussel spasms & burping with uncomfortable, but not painful, weirdness in your throat and chest. This was the main cause of my issue. I'll explain further below.
2: Stress and Anxiety go hand in hand; I found that when I'm stressed it comes around more. In fact, I got hit with both the Acid Reflux and the stressful heart thud at once. It was hell and still is as I sit here.
I recently started drinking raw milk and it was great! Then yesterday, at lunch, I had cereal for the second time with some crappy store bought milk my dad likes. I THINK THAT WAS IT!! That possibly caused my Acid Reflux.
Also, I was so stressed. I work for a call center and they have recently been changing a lot of their online features so people have been flooding our telephone lines. Plus, they expect us to answer emails and chats AT THE SAME TIME!! Flu season has been killing our staff numbers too so it's double the work. This was the cause of the stress. Not to mention I work as a free lance designer and I have like 2 HUGE sites to work on when I get home so this equals like 2 hours for relaxing. It's crappy. So I'm learning to control my thinking and just relax. My father says why think about the past or future or worry about a stressful situation that you have NO CONTROL OVER. It makes tons of sense and is totally helping.
I'm broke as hell so I'm not going to go to the ER or doctors because I can't afford it. Plus I pretty much know what is causing it.