What is your heart rate when it is beating fast? Please measure it, and report back.
To me it seems you're describing a textbook example of anxiety and panic.
I don't know how well you are examined but you should see your physician to get some tests run and rule out certain conditions, but again, to me, it seems a description of anxiety couldn't be written more clearly.
The pause and hard thump you occationally feel during anxious times is a premature heartbeat; a completely benign condition. Anxiety is known to provoke those.