Many thin people can see the pulse of the heart in the stomach and chest. There's just not enough fat to hide these things. It is not a cause for worry.
In the absence of high blood pressure, the *awareness* of a pounding heart is usually a sign of hyper-vigilance to bodily sensations. This is another way of referring to anxiety.
You do apparently have tachycardia, for your heart rate is higher than usual when resting, and goes quite high when you're standing. Have you have a tilt test to check for dysautonomia?
So much is left out of this account. You say your doctor says there's 'another underlying condition.' What IS it? That's a really important part of the puzzle, and almost certainly, somebody who has examined you has a pretty good idea of what it might be.
You don't say what kind of heart monitor you're wearing, or for how long, but clearly it should be some kind of long-term arrangement to catch intermittent episodes.
In light of your complaint of 'vein' and chest pain when your heart races, it seems obvious that you should also have had echo and stress tests of your heart, but you do not mention that. Have these tests been done? If not, why not?
Your comments on another topic indicate that you are familiar with things like tests of thyroid and adrenal function, which should have been employed in your case. If so, what were those results?
More information is needed.