I can't even think about tendons... Where can I post this to find out what this could be?

Something happens to me where sometimes I like stop breathing, and my heart beats so hard when I think of that part in wrists that tenses up and shows through the skin. Veins have the same effect, but not as bad. it is even hard to write about. my heart is already beating super hard. it doesn't happen all the time, but when it does happen it is really bad, and I cannot do anything that will tense it up. I can't even do things that I think will tense it up. I don't know if it is as bad with heartbeats as before, but once in grade school we had to measure our heartbeat from our wrists and neck, and i felt like i was gonna like throw up.

Where can I post this to find out what this could be?