Stress
Working as a medical assistant isn't quite as stressful as driving white-knuckled through a river of lava, but it comes close.
There will be days when you'll be so busy, you'll forget to eat. There will be days where the patients will act like the demon horde, and the doctors will act like they've been collectively possessed by Satan himself. There will be days where you'll screw up a test or a piece of paperwork and some physician's wrath will descend upon your head.
Take a deep breath, and keep on keepin' on.