I have noticed if i get a shock, eg, being woke up by loud thunder in the night, when i feel my pulse i notice it has slowed right down like it is going to stop & then it starts racing like mad, almost like my blood pressure went right down & it's racing to bring it back up to normal levels? Is this normal or is something wrong with my heart?

Shock has a fast thready pulse This is normal, the surprise slows the heart while your body injects adrenalin which causes the heart to speed up.

The adrenalin and speeding heart is the "Fight or Flight" reflex.

But if you want to see some thing else that is neat, if you have a heart monitor. Take a deep breath and hold it as long as you can and watch the heart monitor. At first the heart slows right down, then after many seconds starts speeding up. I used that to fool unsophisticated lie detector at science fair.