What Causes Nails Clubbing?

At first i thought my nails are normal but when i entered nursing i found out that it looks like clubbing. my father has the same nails. even beau's line are present at times. im 21 yrs old. i do notice that i have prominent veins in arms and legs but not that of varicosities.. i also experience getting a "hard on" frequently.. does this suggest having a heart problem?? yet i have passed any medical examinations that i get before college... thanks