Just a little embarrassment.
You will blush, your face turns red, your heart pumps faster, you startle upon your speech or without you knowing it you will tend to avoid a direct eye contact with that person.
How often does one feels that way ?
When you are nervous ? Feeling not-so-good about yourself ?
Or simply, when you are being all lovey-dovey ?
That's when you are seeing him face to face. What if when he's out of sight but not out of mind yet, will you still be feeling all shy about it ?
Indeed, a feeling that is in-controllable and an expression that will just come voluntarily.